Package 'freealg' reference manual

Title:	The Free Algebra
Description:	The free algebra in R with non-commuting indeterminates. Uses 'disordR' discipline (Hankin, 2022, <doi:10.48550/ARXIV.2210.03856>). To cite the package in publications please use Hankin (2022) <doi:10.48550/ARXIV.2211.04002>.
Authors:	Robin K. S. Hankin [aut, cre] (ORCID: <https://orcid.org/0000-0001-5982-0415>)
Maintainer:	Robin K. S. Hankin <[email protected]>
License:	GPL (>= 2)
Version:	1.2-2
Built:	2026-05-29 09:49:51 UTC
Source:	https://github.com/robinhankin/freealg

The Free Algebra

Description

The free algebra in R with non-commuting indeterminates. Uses 'disordR' discipline (Hankin, 2022, <doi:10.48550/ARXIV.2210.03856>). To cite the package in publications please use Hankin (2022) <doi:10.48550/ARXIV.2211.04002>.

Details

The DESCRIPTION file:

Package:	freealg
Type:	Package
Title:	The Free Algebra
Version:	1.2-2
Authors@R:	person(given=c("Robin", "K. S."), family="Hankin", role = c("aut","cre"), email="[email protected]", comment = c(ORCID = "0000-0001-5982-0415"))
Maintainer:	Robin K. S. Hankin <[email protected]>
Depends:	R (>= 4.2)
Description:	The free algebra in R with non-commuting indeterminates. Uses 'disordR' discipline (Hankin, 2022, <doi:10.48550/ARXIV.2210.03856>). To cite the package in publications please use Hankin (2022) <doi:10.48550/ARXIV.2211.04002>.
License:	GPL (>= 2)
LazyData:	yes
Imports:	Rcpp (>= 1.0-7), partitions (>= 1.9-22), disordR (>= 0.9-5-1), methods
LinkingTo:	Rcpp
Suggests:	knitr, testthat, magrittr, markdown, rmarkdown, covr
VignetteBuilder:	knitr
URL:	https://github.com/RobinHankin/freealg, https://robinhankin.github.io/freealg/
BugReports:	https://github.com/RobinHankin/freealg/issues
RoxygenNote:	7.3.3
Encoding:	UTF-8
Config/pak/sysreqs:	libgmp3-dev
Repository:	https://robinhankin.r-universe.dev
Date/Publication:	2026-05-29 07:19:04 UTC
RemoteUrl:	https://github.com/robinhankin/freealg
RemoteRef:	HEAD
RemoteSha:	078f007b3fb560b624cdfe8df8771bc565d88060
Author:	Robin K. S. Hankin [aut, cre] (ORCID: <https://orcid.org/0000-0001-5982-0415>)

Index of help topics:

abelianize              Abelianize a 'freealg' object
accessor                Accessor methods for freealg objects
adjoint                 The adjoint map
constant                The constant term
deriv                   Differentiation of 'freealg' objects
dot-class               Class "dot"
drop                    Drop redundant information
freealg                 The free algebra
freealg-class           Class "freealg"
freealg-package         The Free Algebra
grade                   The grade (or degree) of terms in a 'freealg'
                        object
horner                  Horner's method
inverse                 Inverses
letters                 Single-letter symbols
linear                  Simple free algebra objects
nterms                  Number of terms in a freealg object
Ops.freealg             Arithmetic Ops methods for the the free algebra
pepper                  Combine variables in every possible order
print.freealg           Print freealg objects
rfalg                   Random free algebra objects
subs                    Substitution
zero                    The zero algebraic object

Author(s)

Robin K. S. Hankin [aut, cre] (ORCID: <https://orcid.org/0000-0001-5982-0415>)

Maintainer: Robin K. S. Hankin <[email protected]>

Examples


a <- as.freealg("x+xyx")
b <- as.freealg("4x +XyX")  # upper-case interpreted as inverse

a+b
stopifnot(a+b == b+a)   # should be TRUE

a*b == b*a # FALSE; noncommutative algebra

as.freealg("1+X+xy")^3

rfalg()
rfalg()^2

a <- as.freealg("x+xyx")
b <- as.freealg("4x +XyX")  # upper-case interpreted as inverse

a+b
stopifnot(a+b == b+a)   # should be TRUE

a*b == b*a # FALSE; noncommutative algebra

as.freealg("1+X+xy")^3

rfalg()
rfalg()^2

Abelianize a `freealg` object

Description

Function abelianize() returns a freealg object that is equivalent to its argument under assumption of Abelianness. The symbols are placed in alphabetical order.

Usage

abelianize(x)abelianize(x)

Arguments

x

A freealg object

Details

Abelianizing a free group element means that the symbols can commute past one another. Abelianization is vectorized.

Value

Returns an object of class freealg.

Note

There is a very similar function in the freegroup package. However, the frab package is the best way to work with the free Abelian group.

Author(s)

Robin K. S. Hankin

Examples


abelianize(as.freealg("ba + 2abbba + 3abAB"))

abelianize(.[rfalg(), rfalg()])

abelianize(as.freealg("ba + 2abbba + 3abAB"))

abelianize(.[rfalg(), rfalg()])

Accessor methods for freealg objects

Description

Accessor methods for free algebra objects

Usage

words(x)
coeffs(x, drop=TRUE)
coeffs(x) <- value
words(x)
coeffs(x, drop=TRUE)
coeffs(x) <- value

Arguments

x

Object of class freealg

value

Numeric vector of length 1

drop

Boolean, with default TRUE meaning to drop the disord attribute, as per disordR::drop() and FALSE meaning to consistently return a disord object irregardless

Details

Access or set the different parts of a freealg object. The constant term is technically a coefficient but is documented under constant.Rd.

“Pure” extraction and replacement (as in a[i] and a[i] <- value is implemented experimentally. The code for extraction is cute but not particularly efficient.

Note

There is an extended discussion of disordR discipline in the context of algebras in the mvp package at accessor.Rd.

Author(s)

Robin K. S. Hankin

Examples

a <- rfalg()
a
coeffs(a)
words(a)  # NB: hash is identical to that of coeffs(a)

coeffs(a) <- 7   # replacement methods work 
a
coeffs(a)

a <- rfalg()
a
coeffs(a)
words(a)  # NB: hash is identical to that of coeffs(a)

coeffs(a) <- 7   # replacement methods work 
a
coeffs(a)

The adjoint map

Description

The adjoint $\mathrm{ad}_X$ of $X$ is a map from a Lie group $G$ to the endomorphism group of $G$ defined by

$\mathrm{ad}_X(Y)=\left[X,Y\right]$

Usage

ad(x)
ad(x)

Arguments

x

Object nominally of class freealg but other classes accepted where they make sense

Note

Vignette adjoint gives more description

Author(s)

Robin K. S. Hankin

Examples


x <- rfalg()
y <- rfalg()

f <- ad(x)
f(y)

f(f(y)) # [x, [x, y]]

x <- rfalg()
y <- rfalg()

f <- ad(x)
f(y)

f(f(y)) # [x, [x, y]]

The constant term

Description

Get and set the constant term of a freealg object

Usage

## S3 method for class 'freealg'
constant(x)
## S3 method for class 'numeric'
constant(x)
## S3 replacement method for class 'freealg'
constant(x) <- value
is.constant(x)
## S3 method for class 'freealg'
constant(x)
## S3 method for class 'numeric'
constant(x)
## S3 replacement method for class 'freealg'
constant(x) <- value
is.constant(x)

Arguments

x

Object of class freealg

value

Scalar value for the constant

Details

The constant term in a free algebra object is the coefficient of the empty term. In a freealg object, the map including $\emptyset\longrightarrow v$ implies that $v$ is the constant.

If x is a freealg object, constant(x) returns the value of the constant in the multivariate polynomial; if x is numeric, it returns a constant freealg object with value x.

Function is.constant() returns TRUE if its argument has no variables and FALSE otherwise.

Setting the coefficients of the empty freealg returns the zero (empty) object.

Author(s)

Robin K. S. Hankin

Examples

p <- as.freealg("1+X+Y+xy")

constant(p)
constant(p^5)

constant(p) <- 1000
p

p <- as.freealg("1+X+Y+xy")

constant(p)
constant(p^5)

constant(p) <- 1000
p

Differentiation of `freealg` objects

Description

Differentiation of freealg objects

Usage

## S3 method for class 'freealg'
deriv(x, r, ...)
## S3 method for class 'freealg'
deriv(x, r, ...)

Arguments

x

Object of class freealg

r

Integer vector. Elements denote variables to differentiate with respect to. If r is a character vector, it is interpreted as a=1, b=2, and so on; thus “aab” is interpreted as c(1,1,2)

...

Further arguments, currently ignored

Details

Experimental function deriv(S,v) returns $\frac{\partial^r S}{\partial v_1\partial v_2\ldots\partial v_r}$ , where $v_i$ is v[i]. The Leibniz product rule

$\left(u\cdot v\right)'=uv'+u'v$

operates even if (as here) $u,v$ do not commute. For example, if we wish to differentiate $aaba$ with respect to $a$ , we would write $f(a) = aaba$ and then

$f(a+\delta a) = (a+\delta a)(a+\delta a)b(a+\delta a)$

and working to first order we have

$f(a+\delta a) -f(a)= (\delta a)aba + a(\delta a)ba + aab(\delta a).$

In the package:


    > deriv(as.freealg("aaba"),"a")
    free algebra element algebraically equal to
    + 1*aab(da) + 1*a(da)ba + 1*(da)aba

A term of a freealg object can include negative values which correspond to negative powers of variables. Thus:

    > deriv(as.freealg("AAAA"),"a")       
    free algebra element algebraically equal to
    - 1*AAAA(da)A - 1*AAA(da)AA - 1*AA(da)AAA - 1*A(da)AAAA

(see also the examples). Vector r may include negative integers which mean to differentiate with respect to the inverse of the variable:

    > deriv(as.freealg("3abcbCC"),"C")
    free algebra element algebraically equal to
    + 3*abcbC(dC) + 3*abcb(dC)C - 3*abc(dC)cbCC

It is possible to perform repeated differentiation by passing a suitable value of r. For $\frac{\partial^2}{\partial a\partial c}$ :

    > deriv(as.freealg("aaabAcx"),"ac")
    free algebra element algebraically equal to
    - 1*aaabA(da)A(dc)x + 1*aa(da)bA(dc)x + 1*a(da)abA(dc)x + 1*(da)aabA(dc)x

The infinitesimal indeterminates (“da” etc) are represented by SHRT_MAX+r, where r is the integer for the symbol, and SHRT_MAX is the maximum short integer. This includes negative r. So the maximum number for any symbol is SHRT_MAX. Inverse elements such as A, being represented by negative integers, have differentials that are SHRT_MAX-r.

Function deriv() calls helper function lowlevel_diffn() which is documented at Ops.freealg.Rd.

A vignette illustrating this concept and furnishing numerical verification of the code in the context of matrix algebra is given at inst/freealg_matrix.Rmd.

Author(s)

Robin K. S. Hankin

Examples


deriv(as.freealg("4*aaaabaacAc"),1)

x <- rfalg()
deriv(x, 1:3)

y <- rfalg(7, 7, 17, TRUE)

deriv(y, 1:5) - deriv(y, sample(1:5)) # should be zero

deriv(as.freealg("4*aaaabaacAc"),1)

x <- rfalg()
deriv(x, 1:3)

y <- rfalg(7, 7, 17, TRUE)

deriv(y, 1:5) - deriv(y, sample(1:5)) # should be zero

Class “dot”

Description

The dot object is defined so that .[x,y] returns the commutator of x and y, that is, xy-yx or the Lie bracket $[x,y]$ . It would have been nice to use [x,y] (that is, without the dot) but although this is syntactically consistent, it cannot be done in R.

The “meat” of the dot functionality is:

setClass("dot", slots = c(ignore='numeric'))
`.` <- new("dot")
setMethod("[",signature(x="dot",i="ANY",j="ANY"),function(x,i,j,drop){i*j-j*i})

The package code includes other bits and pieces such as informative error messages for idiom such as .[]. The package defines a matrix method for the dot object. This is because “*” returns (incorrectly, in my view) the elementwise product, not the matrix product.

The Jacobi identity, satisfied by any associative algebra, is

$\left[x,\left[y,z\right]\right]+ \left[y,\left[z,x\right]\right]+ \left[z,\left[x,y\right]\right]=0$

and the left hand side is returned by jacobi(), which should be zero (for some definition of “zero”).

Function ad() returns the adjoint operator. The adjoint vignette provides details and examples of the adjoint operator.

The dot object is generated by running script inst/dot.Rmd, which includes some further discussion and technical documentation, and creates file dot.rda which resides in the data/ directory.

Value

Always returns an object of the same class as xy.

Slots

ignore:: Object of class "numeric", just a formal placeholder

Methods

[: signature(x = "dot", i = "ANY", j = "ANY"): ...
[: signature(x = "dot", i = "ANY", j = "missing"): ...
[: signature(x = "dot", i = "function", j = "function"): ...
[: signature(x = "dot", i = "matrix", j = "matrix"): ...
[: signature(x = "dot", i = "missing", j = "ANY"): ...
[: signature(x = "dot", i = "missing", j = "missing"): ...

Author(s)

Robin K. S. Hankin

Examples


.[as.freealg("x"), as.freealg("y")]
.[as.freealg("x"), as.freealg("y+2z")]
.[as.freealg("x+y+2xYx"), as.freealg("x+y+2xYx")]


x <- rfalg()
y <- rfalg()
z <- rfalg()

jacobi(x,y,z) # Jacobi identity
.[x,.[y,z]] + .[y,.[z,x]] + .[z,.[x,y]]  # Jacobi, expanded


f <- ad(x)
f(y)


rM <- function(...){matrix(sample(1:9, 9), 3, 3)} # a random matrix

M <- rM()
N <- rM()
O <- rM()

.[M, N]
jacobi(M, N, O)

plot(.[sin,tan](seq(from=0, to=1, len=100)))

.[as.freealg("x"), as.freealg("y")]
.[as.freealg("x"), as.freealg("y+2z")]
.[as.freealg("x+y+2xYx"), as.freealg("x+y+2xYx")]


x <- rfalg()
y <- rfalg()
z <- rfalg()

jacobi(x,y,z) # Jacobi identity
.[x,.[y,z]] + .[y,.[z,x]] + .[z,.[x,y]]  # Jacobi, expanded


f <- ad(x)
f(y)


rM <- function(...){matrix(sample(1:9, 9), 3, 3)} # a random matrix

M <- rM()
N <- rM()
O <- rM()

.[M, N]
jacobi(M, N, O)

plot(.[sin,tan](seq(from=0, to=1, len=100)))

Drop redundant information

Description

Coerce constant free algebra objects to numeric

Usage

drop(x)drop(x)

Arguments

x

Free algebra object

Details

If its argument is a constant freealg object, coerce to numeric. Modelled on base::drop().

Note

A few functions in the package take drop as an argument which, if TRUE, means that the function returns a dropped value.

Author(s)

Robin K. S. Hankin

Examples

drop(linear(1:5))
drop(4+linear(1:5)*0)
drop(linear(1:5))
drop(4+linear(1:5)*0)

The free algebra

Description

Create, test for, and coerce to, freealg objects

Usage

freealg(words, coeffs)
is_ok_free(words,coeffs)
is.freealg(x)
as.freealg(x,...)
char_to_freealg(ch)
natural_char_to_freealg(string)
string_to_freealg(string)
vector_to_free(v,coeffs)
freealg(words, coeffs)
is_ok_free(words,coeffs)
is.freealg(x)
as.freealg(x,...)
char_to_freealg(ch)
natural_char_to_freealg(string)
string_to_freealg(string)
vector_to_free(v,coeffs)

Arguments

words

Terms of the algebra object, eg c(1,2,-1,-3,-2) corresponds to abACB because $a=1$ , $b=2$ etc; uppercase, or negative number, means inverse

coeffs

Numeric vector corresponding to the coefficients of each element of the word list

string

Character string

ch

Character vector

v

Vector of integers

x

Object possibly of class freealg

...

Further arguments, passed to the methods

Details

Function freealg() is the formal creation mechanism for freealg objects. However, it is not very user-friendly; it is better to use as.freealg() in day-to-day use (although it does use heuristics for the coefficients if not supplied).

Low-level helper function is_ok_freealg() checks for consistency of its arguments.

A freealg object is a two-element list. The first element is a list of integer vectors representing the indices and the second is a numeric vector of coefficients. Thus, for example:

> as.freealg("a+4bd+3abbbbc")
free algebra element algebraically equal to
 + 1*a + 3*abbbbc + 4*bd
> dput(as.freealg("a+4bd+3abbbbc"))
structure(list(indices = list(1L, c(1L, 2L, 2L, 2L, 2L, 3L), 
    c(2L, 4L)), coeffs = c(1, 3, 4)), class = "freealg")

Observe that the order of the terms is not preserved and indeed is undefined (implementation-specific). Zero entries are stripped out.

Character strings may be coerced to freealg objects; as.freealg() calls natural_char_to_freealg(), which is user-friendly. Functions char_to_freealg() and string_to_freealg() are low-level helper functions: function string_to_freealg() takes a length-one character vector such as "xy" or "-5xxy" and returns the corresponding one-term free algebra object. Function char_to_freealg() takes longer character vectors, and returns the sum [sic] of string_to_freealg() applied to each element.

These functions assume that upper-case letters are the multiplicative inverses of the lower-case equivalents; so for example as.freealg("aA") and as.freealg(aBcCbA) evaluate to one. This can be confusing with the default print method.

Note

Internally, the package uses signed integers and as such can have .Machine$integer.max different symbols; on my machine this is 2147483647. Of course the print method cannot deal with this as it only has 26 symbols for letters a-z (and A-Z for the inverses), but the objects themselves do not care about the print method. Note also that the experimental calculus facility (as per deriv()) reserves numbers in the range SHRT_MAX ${}\pm r$ for infinitesimals, where r is the integer for a symbol. This system might change in the future.

Author(s)

Robin K. S. Hankin

Examples


freealg(list(1:2, 2:1, numeric(0), 1:6), 1:4)
freealg(list(1:2, 2:1, numeric(0), 1:6)) # heuristics for coeffs: assume 1

freealg(sapply(1:5, seq_len), 1:5)

freealg(replicate(5, sample(-5:5,rgeom(1,1/5),replace=TRUE)), 1:5)

as.freealg("1+xaX")^5

freealg(list(1:2, 2:1, numeric(0), 1:6), 1:4)
freealg(list(1:2, 2:1, numeric(0), 1:6)) # heuristics for coeffs: assume 1

freealg(sapply(1:5, seq_len), 1:5)

freealg(replicate(5, sample(-5:5,rgeom(1,1/5),replace=TRUE)), 1:5)

as.freealg("1+xaX")^5

Class “freealg”

Description

The formal S4 class for freealg objects

Objects from the Class

Formal class freealg is used for functions such as drop() which need a S4 object.

Author(s)

Robin K. S. Hankin

The grade (or degree) of terms in a `freealg` object

Description

The free algebra $\mathcal B$ is a graded algebra: that is, for each integer $n\geqslant 0$ there is a homogeneous subspace $\mathcal{B}_n$ with $\mathcal{B}_0=\mathcal{R}$ and

$\mathcal{B}=\bigoplus_{n=0}^\infty\mathcal{B}_n,\quad\mbox{and}\quad\mathcal{B}_n\mathcal{B}_m\subseteq\mathcal{B}_{n+m}\quad\mbox{for all $m,n\geqslant 0.$}$

The elements of $\cup_{n\geqslant 0}\mathcal{B}_n$ are called homogeneous and those of $\mathcal{B}_n$ are called homogeneous of degree (or grade) $n$ .

The grade of a term is the number of symbols in it. Thus the grade of xxx and 4xxy is 3; the grade of a constant is zero. Because the terms are stored in an implementation-specific way, the grade of a multi-term object is a disord object.

The grade of the zero freealg object, grade(as.freealg(0)), is defined to be $-\infty$ , as per Knuth [TAOCP, volume 2, p436]. This ensures that max(grades(abelianize(x))) <= max(grades(x)) is always satisfied. However, a case for NULL could be made.

Usage

grades(x)
grade(x, n, drop=FALSE)
grade(x, n) <- value
deg(x)
grades(x)
grade(x, n, drop=FALSE)
grade(x, n) <- value
deg(x)

Arguments

x

Freealg object

n

Integer vector

value

Replacement value, a numeric vector

drop

Boolean, indicating whether or not to return a dropped value

Details

grades(x) returns the grade (number of symbols) in each term of a freealg object x.

deg(x) returns the maximum of the grades of each symbol of x; max(grades(x)).

grade(x,n) returns the freealg object comprising terms with grade n (which may be a vector). Note that this function is considerably less efficient than clifford::grade().

The drop argument to grade() only makes a difference if either the result is zero, or that n=0 (that is, extracting the constant). It defaults to FALSE on the grounds that constant() returns a dropped value.

grade(x,n) <- value sets the coefficients of terms with grade n. For value, a length-one numeric vector is accepted (notably zero, which kills terms of grade n) and also a freealg object comprising terms of grade n.

Value

Returns a disord object

Note

The clifford package includes a similar concept, also called grade.

Author(s)

Robin K. S. Hankin

References

H. Munthe-Kaas and B. Owren 1999. “Computations in a free Lie algebra”, Phil. Trans. R. Soc. Lond. A, 357:957–981 (theorem 3.8)

Examples



X <- as.freealg("1 -x + 5*y + 6*x*y -8*x*x*x*x*y*x")
X
grades(X)

a <- rfalg(30)
a
grades(a)
grade(a, 2)
grade(a, 2) <- 0 # kill all grade-2 terms
a

grade(a, 1) <- grade(a, 1) * 888 
a
X <- as.freealg("1 -x + 5*y + 6*x*y -8*x*x*x*x*y*x")
X
grades(X)

a <- rfalg(30)
a
grades(a)
grade(a, 2)
grade(a, 2) <- 0 # kill all grade-2 terms
a

grade(a, 1) <- grade(a, 1) * 888 
a

Horner's method

Description

Horner's method for multivariate polynomials

Usage

horner(P, v)
horner(P, v)

Arguments

P

Free algebra polynomial

v

Numeric vector of coefficients

Details

This function is (almost) the same as mvp::horner().

Given a polynomial

$p(x) = a_0 +a_1x+a_2x^2+\cdots + a_nx^n$

it is possible to express $p(x)$ in the algebraically equivalent form

$p(x) = a_0 + x\left(a_1+x\left(a_2+\cdots + x\left(a_{n-1} +xa_n \right)\cdots\right)\right)$

which is more efficient for evaluation, as it requires only $n$ multiplications and $n$ additions, and this is optimal. Function horner() coerces its first argument to class freealg.

Author(s)

Robin K. S. Hankin

Examples


horner("x", 1:4)  # note constant term is 1.

horner("x+y", 1:3) # note presence of xy and yx terms

horner("1+x+xyX", 1:3)
horner("x", 1:4)  # note constant term is 1.

horner("x+y", 1:3) # note presence of xy and yx terms

horner("1+x+xyX", 1:3)

Inverses

Description

Multiplicative inverses of symbols in the free algebra

Usage

all_pos(x)
keep_pos(x)
all_pos(x)
keep_pos(x)

Arguments

x

Freealg object

Details

Function all_pos() tests for its argument having only positive powers (that is, no inverse symbols present); function keep_pos() discards any term with a negative power.

At various points in the package, it is assumed that upper-case letters are the multiplicative inverses of the lower-case equivalents; so for example as.freealg("aA") and as.freealg("aBcCbA") evaluate to one. This can be confusing with the default print method.

Even though individual symbols have multiplicative inverses, a general element of the free algebra will not have a multiplicative inverse. For example, 1+x does not have an inverse. The free algebra is not a division algebra, in general.

Author(s)

Robin K. S. Hankin

Examples


all_pos(rfalg(include.negative = TRUE))
all_pos(rfalg(include.negative = FALSE))

as.freealg("1+xaX")^5

all_pos(rfalg(include.negative = TRUE))
all_pos(rfalg(include.negative = FALSE))

as.freealg("1+xaX")^5

Single-letter symbols

Description

Variables a, b,..., z and their inverses A-Z are given their freealg semantic meaning.

Details

Sometimes it is convenient in an R session to have all 26 letters a-z and all 26 uppercase letters A-Z adopt their free algebra interpretations. To access this, load the lettersymbols dataset, which is provided with the package in the inst directory:

  load(system.file("lettersymbols.rda",package="freealg"))

Executing this allows you to do cool things such as the following:

> (1+a-b^2)^4
free algebra element algebraically equal to
+ 1 + 4a + 6aa + 4aaa + aaaa - aaabb - 4aabb - aabba + aabbbb - 6abb - 4abba -
abbaa + abbabb + 4abbbb + abbbba - abbbbbb - 4bb - 6bba - 4bbaa - bbaaa +
bbaabb + 4bbabb + bbabba - bbabbbb + 6bbbb + 4bbbba + bbbbaa - bbbbabb -
4bbbbbb - bbbbbba + bbbbbbbb
>

Lowercase letters c, q, t, and uppercase letters C, D, F, I, T might pose difficulties.

These objects can also be generated by running script inst/symb.Rmd, which includes some further discussion and technical documentation and creates file lettersymbols.rda which formerly resided in the data/ directory.

Author(s)

Robin K. S. Hankin

Simple free algebra objects

Description

Create simple free algebra objects including linear expressions. For example:

> linear(1:3)
free algebra element algebraically equal to
+ 1*a + 2*b + 3*c
> linear(1:3, power=5)
free algebra element algebraically equal to
+ 1*aaaaa + 2*bbbbb + 3*ccccc
>

Usage

linear(x, power=1)
linear(x, power=1)

Arguments

x

Numeric vector of terms

power

Integer vector of powers

Note

It is instructive to compare the functionality documented here with their mvp equivalents. Many of the functions documented at mvp::special.Rd do not make sense in the context of the free algebra. Function mvp::product(), for example, imposes an order on the expansion.

Function constant() is documented at constant.Rd, but is listed below for convenience.

Author(s)

Robin K. S. Hankin

Examples

linear(1:3)         
linear(1:3, power=5)
linear(1:3, power=3:1)
linear(1:3)         
linear(1:3, power=5)
linear(1:3, power=3:1)

Number of terms in a freealg object

Description

Number of terms in a freealg object; number of coefficients

Usage

nterms(x)nterms(x)

Arguments

x

Freealg object

Value

Returns a non-negative integer

Author(s)

Robin K. S. Hankin

Examples

(a <- freealg(list(1:3,4:7,1:10),1:3))
nterms(a)
nterms(a+1)
nterms(a*0)
(a <- freealg(list(1:3,4:7,1:10),1:3))
nterms(a)
nterms(a+1)
nterms(a*0)

Arithmetic Ops methods for the the free algebra

Description

Arithmetic operators for manipulation of freealg objects such as addition, multiplication, powers, etc

Usage

## S3 method for class 'freealg'
Ops(e1, e2)
free_negative(S)
free_power_scalar(S, n)
free_power_free(e1, e2)
free_eq_free(e1, e2)
free_plus_numeric(S, x)
free_plus_free(e1, e2)
lowlevel_simplify(words, coeffs)
lowlevel_free_prod(words1, coeffs1, words2, coeffs2)
lowlevel_free_sum(words1, coeffs1, words2, coeffs2)
lowlevel_free_power(words, coeffs, n)
lowlevel_diffn(words, coeffs, r)
lowlevel_subs(words1, coeffs1, words2, coeffs2, r)
inv(S)
## S3 method for class 'freealg'
Ops(e1, e2)
free_negative(S)
free_power_scalar(S, n)
free_power_free(e1, e2)
free_eq_free(e1, e2)
free_plus_numeric(S, x)
free_plus_free(e1, e2)
lowlevel_simplify(words, coeffs)
lowlevel_free_prod(words1, coeffs1, words2, coeffs2)
lowlevel_free_sum(words1, coeffs1, words2, coeffs2)
lowlevel_free_power(words, coeffs, n)
lowlevel_diffn(words, coeffs, r)
lowlevel_subs(words1, coeffs1, words2, coeffs2, r)
inv(S)

Arguments

S, e1, e2

Objects of class freealg

n

Integer, possibly non-positive

r

Integer vector indicating variables to differentiate with respect to

x

Scalar value

words, words1, words2

A list of words, that is, a list of integer vectors representing the variables in each term

coeffs, coeffs1, coeffs2

Numeric vector representing the coefficients of each word

Details

The function Ops.freealg() passes binary arithmetic operators (“+”, “-”, “*”, “^”, and “==”) to the appropriate specialist function.

Integer powers are implemented. The caret [e.g., a^n with integer n] denotes arithmetic exponentiation, as in x^3==x*x*x. As an experimental feature, this is (sort of) vectorised: if v is a vector of integers, then a^v returns the sum of a raised to the power of each element of v. For example, a^c(n1,n2,n3) is a^n1 + a^n2 + a^n3. Internally, n is tabulated in the interests of efficiency, so a^c(0,2,5,5,5,) = 1 + a^2 + 3a^5 is evaluated with only a single fifth power. Similar functionality is implemented in the mvp package. Beware of bracketing issues: a^1:3, for example, is not defined (use a^(1:3)).

Negative powers, as in a^n with $n<0$ are implemented: if a has an inverse, then a^n == inv(a)^(-n); so a^(n+m)=(a^n)*(a^m) for n,m any integers, positive or negative.

If x and a are freealg objects, then x^a implements group-theoretic conjugation. If a has an inverse, then x^a == inv(a)*x*a. The notation is motivated by the identities x^(ab)=(x^a)^b and (x*y)^a=(x^a)*(y^a).

The only comparison operators are equality and inequality; x==y is defined as is.zero(x-y).

Functions lowlevel_foo() are low-level functions that interface directly with the C routines in the src/ directory and are not intended for the end-user.

Function inv() is defined only for freealg objects with a single term. If x has a single term we have inv(x)*x=x*inv(x)=1. There is no corresponding division in the package because a/b may be either a*inv(b) or inv(b)*a.

Author(s)

Robin K. S. Hankin

Examples

rfalg()
as.freealg("1+x+xy+yx")  # variables are non-commutative
as.freealg("x") * as.freealg("X") # upper-case letters are lower-case inverses

constant(as.freealg("x+y+X+Y")^6)  # OEIS sequence A035610
nterms  (as.freealg("x+y+X+Y")^6)  # OEIS sequence A003462 

inv(as.freealg("2aaabAAAAx"))

as.freealg("a")^(1:7)

x <- rfalg()
y <- rfalg()
a <- as.freealg("a")
b <- as.freealg("b")


(x*y)^a == x^a * y^a
x^(a*b) == (x^a)^b


rfalg()
as.freealg("1+x+xy+yx")  # variables are non-commutative
as.freealg("x") * as.freealg("X") # upper-case letters are lower-case inverses

constant(as.freealg("x+y+X+Y")^6)  # OEIS sequence A035610
nterms  (as.freealg("x+y+X+Y")^6)  # OEIS sequence A003462 

inv(as.freealg("2aaabAAAAx"))

as.freealg("a")^(1:7)

x <- rfalg()
y <- rfalg()
a <- as.freealg("a")
b <- as.freealg("b")


(x*y)^a == x^a * y^a
x^(a*b) == (x^a)^b

Combine variables in every possible order

Description

Given a list of variables, construct every term comprising only those variables; function pepper() returns a free algebra object equal to the sum of these terms.

The function is named for a query from an exam question set by Sarah Marshall in which she asked how many ways there are to arrange the letters of word “pepper”, the answer being $\left({6\atop 1\,2\,3}\right)=\frac{6!}{1!2!3!}=60$ .

Function multiset() in the partitions package gives related functionality; for the record, one way to reproduce pepper("pepper") would be

    apply(matrix(c("p","e","r")[multiset(c(1,1,1,2,2,3))],nrow=6),2,paste,collapse="")

Usage

pepper(v)
pepper(v)

Arguments

v

Variables to combine. If a character string, coerce to variable numbers

Author(s)

Robin K. S. Hankin

Examples

pepper(c(1, 1, 1, 1, 1, 1, 2))  # 6 a's and 1 b
pepper(c(1, 2, 2, 2, 3))        # 1 a, 3 b's and 1 c
pepper("pepper")
pepper(c(1, 1, 1, 1, 1, 1, 2))  # 6 a's and 1 b
pepper(c(1, 2, 2, 2, 3))        # 1 a, 3 b's and 1 c
pepper("pepper")

Print freealg objects

Description

Print methods for free algebra objects. The indeterminates are represented using lowercase letters a-z (currently hard coded).

Usage

## S3 method for class 'freealg'
print(x,...)
## S3 method for class 'freealg'
print(x,...)

Arguments

x

Object of class freealg in the print method

...

Further arguments, currently ignored

Note

The print method does not change the internal representation of a freealg object, which is a two-element list, the first of which is a list of integer vectors representing words, and the second is a numeric vector of coefficients.

The print method uses lowercase letters a-z to represent the indeterminates; this is currently hard coded:

> (x <- as.freealg("6abbbc + 7cax"))
free algebra element algebraically equal to
+ 6*abbbc + 7*cax
> unclass(x)
$indices
$indices[[1]]
[1] 1 2 2 2 3

$indices[[2]]
[1]  3  1 24


$coeffs
[1] 6 7

The print method has special dispensation for length-zero freealg objects but these are not handled entirely consistently.

The print method is sensitive to the value of getOption("usecaret"), defaulting to “FALSE”. The default is to use uppercase letters to represent multiplicative inverses. Thus, the inverse of a appears as either “a^-1” if usecaret is TRUE, and “A” if FALSE. Carets become cumbersome for powers above the first. For example, the default notation for $aba^{-2}$ is abAA but becomes aba^-1a^-1 if usecaret is TRUE.

The symbols for the indeterminates are currently hardcoded as c(letters, LETTERS). The intent is to be able to signify 52 distinct indeterminates, a-z, A-Z. This works fine if option usecaret is TRUE. But if option usecaret is FALSE, this can be confusing: for example, indeterminate number 1 appears as a, and its inverse would appear as “A”. But indeterminate number 27 also appears as “A”. They look the same, but no warning is given: caveat emptor!

The method is also sensitive to getOption("mulsym"), defaulting to NULL. This is the multiplication symbol used between the coefficient and the indeterminate string. Sometimes an asterisk, * or a space, might be useful. If mulsym takes its default of NULL [or a length zero string], the print method suppresses coefficients of $\pm 1$ .

Integers exceeding SHRT_MAX are reserved for infinitesimals, which are printed as “da”; see the note at deriv.Rd for details.

Author(s)

Robin K. S. Hankin

Examples


rfalg()

x <- rfalg(inc=TRUE)
x                           # default
options("usecaret" = TRUE)  # use caret
x
options("usecaret" = FALSE) # back to the default
x


x <- freealg(list(5,1:4,3,8,7), c(1,1,1,3,22))
x


options(mulsym = "*")
x
options(mulsym = NULL)  # restore default

rfalg()

x <- rfalg(inc=TRUE)
x                           # default
options("usecaret" = TRUE)  # use caret
x
options("usecaret" = FALSE) # back to the default
x


x <- freealg(list(5,1:4,3,8,7), c(1,1,1,3,22))
x


options(mulsym = "*")
x
options(mulsym = NULL)  # restore default

Random free algebra objects

Description

Generate random elements of the free algebra, intended as quick “get you going” examples of freealg objects

Usage

rfalg(n=7, distinct=3, maxsize=4, include.negative=FALSE)
rfalgg(n=30, distinct=8, maxsize=7, include.negative=FALSE)
rfalggg(n=100, distinct=26, maxsize=30, include.negative=FALSE)
rfalg(n=7, distinct=3, maxsize=4, include.negative=FALSE)
rfalgg(n=30, distinct=8, maxsize=7, include.negative=FALSE)
rfalggg(n=100, distinct=26, maxsize=30, include.negative=FALSE)

Arguments

n

Number of terms to generate

distinct

Number of distinct symbols to use

maxsize

Maximum number of symbols in any word

include.negative

Boolean, with default FALSE meaning to use only positive symbols (lower-case letters) and TRUE meaning to use upper-case letters as well, corresponding to the inverse of the lower-case symbols

Details

Terms are automatically simplified; for example, aaBaAbaC will be simplified to aaaC.

Functions rfalgg() and rfalggg() return successively more complicated freealg objects.

Author(s)

Robin K. S. Hankin

Examples

rfalg()
rfalg(include.negative = TRUE)^2

constant(rfalg())

rfalg()
rfalg(include.negative = TRUE)^2

constant(rfalg())

Substitution

Description

Substitute symbols in a freealg object for numbers or other freealg objects

Usage

subs(...)
subsu(S1, S2, r)
subs(...)
subsu(S1, S2, r)

Arguments

S1, S2

Objects of class freealg

r

Integer specifying symbol to substitute ( $a=1,b=2$ etc)

...

First element is a freealg object, remainder are named arguments corresponding to variables to substitute

Details

Function subs() substitutes variables for freealg objects (coerced if necessary) using natural R idiom. Observe that this type of substitution is sensitive to order:

> subs("ax",a="1+x",x="1+a")
free algebra element algebraically equal to
 + 2 + 3*a + 1*aa

> subs("ax",x="1+a",a="1+x")
free algebra element algebraically equal to
 + 2 + 3*x + 1*xx

Functions subsu() is a lower-level formal function, not really intended for the end-user. Function subsu() takes S1 and substitutes occurrences of symbol r with S2.

No equivalent to mvp::subvec() is currently implemented.

Value

Returns a freealg object.

Note

Function subs() is one place in the package where the use of letters is effectively hard-wired in. Idiom such as

subs("abccc",b="1+3x")

is very nice, but identifies “b” with 2. Note that argument r of subsu() is canonically an integer but a single character is interpreted as a letter. See also the note at freealg.Rd.

Author(s)

Robin K. S. Hankin

Examples

subs("abccc", b="1+3x")
subs("aaaa", a="1+x")  # binomial

subs("abA", b=31)

subs("1+a", a="A")   # can substitute for an inverse
subs("A", a="1+x")   # inverses are not substituted for

a <- rfalg()
subs(a, a=a)


## Sequential substitution works:

subs("abccc", b="1+3x", x="1+d+2e")
subs(rfalg(), a=rfalg())

subs("abccc", b="1+3x")
subs("aaaa", a="1+x")  # binomial

subs("abA", b=31)

subs("1+a", a="A")   # can substitute for an inverse
subs("A", a="1+x")   # inverses are not substituted for

a <- rfalg()
subs(a, a=a)


## Sequential substitution works:

subs("abccc", b="1+3x", x="1+d+2e")
subs(rfalg(), a=rfalg())

The zero algebraic object

Description

Test for a freealg object's being zero

Usage

is.zero(x)
is.zero(x)

Arguments

x

Object of class freealg

Details

Function is.zero() returns TRUE if x is indeed the zero free algebra object. It is defined as length(coeffs(x)) == 0 for reasons of efficiency, but conceptually it returns x == constant(0).

(Use constant(0) to create the zero object).

Author(s)

Robin K. S. Hankin

Examples


stopifnot(is.zero(constant(0)))

stopifnot(is.zero(constant(0)))

Package 'freealg'

Help Index

The Free Algebra

Description

Details

Author(s)

Examples

Abelianize a freealg object

Description

Usage

Arguments

Details

Value

Note

Author(s)

Examples

Accessor methods for freealg objects

Description

Usage

Arguments

Details

Note

Author(s)

See Also

Examples

The adjoint map

Description

Usage

Arguments

Note

Author(s)

Examples

The constant term

Description

Usage

Arguments

Details

Author(s)

Examples

Differentiation of freealg objects

Description

Usage

Arguments

Details

Author(s)

Examples

Class “dot”

Description

Value

Slots

Methods

Author(s)

Examples

Drop redundant information

Description

Usage

Arguments

Details

Note

Author(s)

See Also

Examples

The free algebra

Description

Usage

Arguments

Details

Note

Author(s)

Examples

Class “freealg”

Description

Objects from the Class

Author(s)

The grade (or degree) of terms in a freealg object

Description

Usage

Arguments

Details

Value

Abelianize a `freealg` object

Differentiation of `freealg` objects

The grade (or degree) of terms in a `freealg` object