R expressions

A call to yacas may be in the form of an R expression which involves valid R calls, symbols or constants (though not all valid R expressions are valid). For example:

exp1 <- yacas(expression(Factor(x^2-1)))

The result exp1 is not an expression in the R sense but an object of class "yacas". To evaluate the resulting expression numerically, we can do

Eval(exp1, list(x=4))

## [1] 15

yacas expressions

Some commands are not proper R expressions. For example, typing

yacas(expression(D(x)Sin(x)))

produces an error. For such cases we can make a specification using the yacas syntax:

yacas("D(x)Sin(x)")

## yacas_expression(cos(x))

Sym objects

Probably the most elegant way of working with yacas is by using Sym objects. A Sym object is a yacas character string that has the “Sym” class. One can combine Sym objects with other Sym objects as well as to other R objects using +, - and other similar R operators.

The function Sym(x) coerces an object x to a Sym object by first coercing it to character and then changing its class to “Sym”:

x <- Sym("x")

Operations on Sym objects lead to new Sym objects:

x+4

## yacas_expression(x + 4)

One can apply sin, cos, tan, deriv, Integrate and other provided functions to Sym objects. For example:

Integrate(sin(x), x)

## yacas_expression(-cos(x))

In this way the communication with yacas is ``tacit’’.

It is important to note the difference between the R name x and the symbol "x" as illustrated below:

x <- Sym("xs")
x

## yacas_expression(xs)

x+4

## yacas_expression(xs + 4)

Eval(x+4, list(xs=5))

## [1] 9

The convention in the following is 1) that Sym objects match with their names that they end with an ‘s’, e.g.

xs <- Sym('xs')

Setting and clearing a variable

The function Set() and the operator := can both be used to assign values to global variables.

yacas("n := (10 + 2) * 5")

## yacas_expression(60)

yacas("n := n+n")

## yacas_expression(120)

#yacas("Set(z, Cos(a))")
#yacas("z+z")

The same can be achieved with Sym objects: Consider:

Set(ns, (10 + 2) * 5)

## yacas_expression(60)

Now ns exists as a variable in yacas (and we can make computations on this variable as above). However we have no handle on this variable in R. Such a handle is obtained with

ns <- Sym("ns")

Now the R variable ns refers to the yacas variable ns and we can make calculations directly from R, e.g:

Set(ns,123)

## yacas_expression(123)

ns

## yacas_expression(123)

ns^2

## yacas_expression(15129)

Likewise:

as <- Sym("as")
zs <- Sym("zs")
Set(zs, cos(as))

## yacas_expression(cos(as))

zs + zs

## yacas_expression(2 * cos(as))

Clear a variable binding execute Clear():

yacas(expression(n))

## yacas_expression(120)

yacas("Clear(n)")

## yacas_expression(TRUE)

yacas(expression(n))

## yacas_expression(n)

Set(ns, 1)

## yacas_expression(1)

ns <- Sym("ns")
ns

## yacas_expression(1)

Clear(ns)

## yacas_expression(TRUE)

ns

## yacas_expression(ns)

Symbolic and numerical evaluations, precision

Evaluations are generally exact:

yacas("Exp(0)")

## yacas_expression(1)

yacas("Exp(1)")

## yacas_expression(exp(1))

yacas("Sin(Pi/4)")

## yacas_expression(root(1/2, 2))

yacas("355/113")

## yacas_expression(355/113)

exp(Sym(0))

## yacas_expression(1)

exp(Sym(1))

## yacas_expression(exp(1))

sin(Pi/4)

## yacas_expression(root(1/2, 2))

Sym("355/113")

## yacas_expression(355/113)

To obtain a numerical evaluation (approximation), the N() function can be used:

yacas("N(Exp(1))")

## yacas_expression(2.7182818284)

yacas("N(Sin(Pi/4))")

## yacas_expression(0.7071067811)

yacas("N(355/113)")

## yacas_expression(3.1415929203)

N(exp(1))

## yacas_expression(2.71828182845905)

N(sin(Pi/4))

## yacas_expression(0.7071067811)

N(355/113)

## yacas_expression(3.14159292035398)

The N() function has an optional second argument, the required precision:

yacas("N(355/133,20)")

## yacas_expression(2.66917293233083)

N("355/113",20) 

## yacas_expression(3.14159292035398)

The command Precision(n) can be used to specify that all floating point numbers should have a fixed precision of n digits:

yacas("Precision(5)")

## yacas_expression(Precision(5))

yacas("N(355/113)")

## yacas_expression(3.1415929203)

Precision(5)

## yacas_expression(Precision(5))

N("355/113")

## yacas_expression(3.1415929203)

Rational numbers

Rational numbers will stay rational as long as the numerator and denominator are integers:

yacas(expression(55/10))

## yacas_expression(11/2)

Sym("55 / 10")

## yacas_expression(11/2)

Symbolic calculation

Some exact manipulations :

yacas("1/14+5/21*(30-(1+1/2)*5^2)")

## yacas_expression(-12/7)

yacas("0+x")

## yacas_expression(x)

yacas("x+1*y")

## yacas_expression(x + y)

yacas("Sin(ArcSin(alpha))+Tan(ArcTan(beta))")

## yacas_expression(alpha + beta)

Sym("1/14+5/21*(1*30-(1+1/2)*5^2)")

## yacas_expression(-12/7)

xs <- Sym("xs")
ys <- Sym("ys")
0+xs

## yacas_expression(xs)

xs+1*ys

## yacas_expression(xs + ys)

sin(asin(xs))+tan(atan(ys))

## yacas_expression(xs + ys)

Complex numbers and the imaginary unit

The imaginary unit \(i\) is denoted I and complex numbers can be entered as either expressions involving I or explicitly Complex(a,b) for a+ib.

yacas("I^2")

## yacas_expression(-1)

yacas("7+3*I")

## yacas_expression(complex_cartesian(7, 3))

yacas("Conjugate(%)")

## yacas_expression(complex_cartesian(7, -3))

yacas("Exp(3*I)")

## yacas_expression(complex_cartesian(cos(3), sin(3)))

I^2

## yacas_expression(-1)

7+3*I

## yacas_expression(complex_cartesian(7, 3))

Conjugate(7+3*I)

## yacas_expression(complex_cartesian(7, -3))

exp(3*I)

## yacas_expression(complex_cartesian(cos(3), sin(3)))

Recall the most recent line – the % operator

The operator % automatically recalls the result from the previous line.

yacas("(1+x)^3")

## yacas_expression((x + 1)^3)

yacas("%")

## yacas_expression((x + 1)^3)

yacas("z:= %")

## yacas_expression((x + 1)^3)

(1+x)^3

## yacas_expression((xs + 1)^3)

zs <- Sym("%")
zs

## yacas_expression((xs + 1)^3)

Printing with PrettyForm, PrettyPrint and TeXForm

There are different ways of displaying the output.

yacas("A:={{a,b},{c,d}}")

## Yacas matrix:
##      [,1] [,2]
## [1,] a    b   
## [2,] c    d

yacas("B:= (1+x)^2+k^3")

## yacas_expression((x + 1)^2 + k^3)

yacas("A")

## Yacas matrix:
##      [,1] [,2]
## [1,] a    b   
## [2,] c    d

yacas("B")

## yacas_expression((x + 1)^2 + k^3)

as <- Sym("as"); bs <- Sym("bs"); cs <- Sym("cs"); ds <- Sym("ds")
A <- List(List(as,bs), List(cs,ds))
ks <- Sym("ks")
B <- (1+xs)^2+ks^3
A

## Yacas matrix:
##      [,1] [,2]
## [1,] as   bs  
## [2,] cs   ds

B

## yacas_expression((xs + 1)^2 + ks^3)

yacas("PrettyForm(A)")

## 
## /              \
## | ( a ) ( b )  |
## |              |
## | ( c ) ( d )  |
## \              /

yacas("PrettyForm(B)")

## 
##          2    3
## ( x + 1 )  + k

PrettyForm(A)

## 
## /                \
## | ( as ) ( bs )  |
## |                |
## | ( cs ) ( ds )  |
## \                /

PrettyForm(B)

## 
##           2     3
## ( xs + 1 )  + ks

yacas("TeXForm(B)")

## [1] "\\left( x + 1\\right)  ^{2} + k ^{3}"

TeXForm(B)

## [1] "\\left( xs + 1\\right)  ^{2} + ks ^{3}"

Factorial

yacas("40!")

## yacas_expression(8.15915283247898e+47)

Factorial(40)

## yacas_expression(Factorial(40))

Taylor expansions

Expand \(\exp(x)\) in three terms around \(0\) and \(a\):

yacas("Taylor(x,0,3) Exp(x)")

## yacas_expression(x + x^2/2 + x^3/6 + 1)

yacas("Taylor(x,a,3) Exp(x)")

## yacas_expression(exp(a) + exp(a) * (x - a) + (x - a)^2 * exp(a)/2 + (x - a)^3 * exp(a)/6)

xs <- Sym("xs")
Taylor(exp(xs),xs,0,3)

## yacas_expression(xs + xs^2/2 + xs^3/6 + 1)

as <- Sym("as")
Taylor(exp(x),x,as,3)

## yacas_expression(exp(as) + exp(as) * (xs - as) + (xs - as)^2 * exp(as)/2 + (xs - as)^3 * exp(as)/6)

The InverseTaylor() function builds the Taylor series expansion of the inverse of an expression. For example, the Taylor expansion in two terms of the inverse of \(\exp(x)\) around \(x=0\) (which is the Taylor expansion of \(Ln(y)\) around \(y=1\)):

yacas("InverseTaylor(x,0,2)Exp(x)")

## yacas_expression(x - 1 - (x - 1)^2/2)

yacas("Taylor(y,1,2)Ln(y)")

## yacas_expression(y - 1 - (y - 1)^2/2)

InverseTaylor(exp(xs),xs,0,2)

## yacas_expression(xs + xs^2/2 + 1)

Taylor(log(ys),ys,1,2)

## yacas_expression(ys - 1 - (ys - 1)^2/2)

Solving equations

yacas("Solve(x/(1+x) == a, x)")

## Yacas vector:
## [1] x == a/(1 - a)

yacas("Solve(x^2+x == 0, x)")

## Yacas vector:
## [1] x == 0  x == -1

Solve(xs/(1+xs) == as, xs)

## Yacas vector:
## [1] xs == as/(1 - as)

Solve(xs^2+xs == 0, xs)

## Yacas vector:
## [1] xs == 0  xs == -1

Solve(List(xs^2+ys^2==6, xs-ys==3), List(xs,ys))

## Yacas matrix:
##      [,1]                    [,2]    
## [1,] xs == root(6 - ys^2, 2) ys == ys

# FIXME
#Solve(List(mean==(xs/(xs+ys)), variance==((xs*ys)/(((xs+ys)^2) * (xs+ys+1)))),
#    List(xs,ys))

yacas("Newton(Sin(x)-Exp(x),x, 0.5, 0.0001)")

## yacas_expression(-3.1830630118)

Newton(sin(xs)-exp(xs),xs, 0.5, 0.0001) 

## yacas_expression(-3.1830630118)

Expanding polynomials

yacas(expression(Expand((1+x)^3)))

## yacas_expression(x^3 + 3 * x^2 + 3 * x + 1)

Expand((x+1)^3)

## yacas_expression(xs^3 + 3 * xs^2 + 3 * xs + 1)

Simplifying an expression

The function Simplify() attempts to reduce an expression to a simpler form.

y <- Sym("y")
yacas("(x+y)^3-(x-y)^3")

## yacas_expression((x + y)^3 - (x - y)^3)

yacas("Simplify(%)")

## yacas_expression(6 * (x^2 * y) + 2 * y^3)

(x+y)^3-(x-y)^3

## yacas_expression((xs + y)^3 - (xs - y)^3)

Simplify("%")

## yacas_expression(6 * (xs^2 * y) + 2 * y^3)

Analytical derivatives

Analytical derivatives of functions can be evaluated with the D() and deriv() functions:

yacas("D(x) Sin(x)")

## yacas_expression(cos(x))

deriv(sin(xs), xs)

## yacas_expression(cos(xs))

These functions also accepts an argument specifying how often the derivative has to be taken, e.g:

yacas("D(x,4)Sin(x)")

## yacas_expression(sin(x))

deriv(sin(xs),xs,4)

## yacas_expression(sin(xs))

Integration

@ <<echo=F,results=hide>>= yacas(“Clear(a,b,A,B)”)

```r
#yacas("Integrate(x,a,b)Sin(x)")
#yacas("Integrate(x,a,b)Ln(x)+x")
#yacas("Integrate(x)1/(x^2-1)")
yacas("Integrate(x)Sin(a*x)^2*Cos(b*x)")

## yacas_expression((2 * sin(b * x)/b - (sin(-2 * (x * a) - b * x)/(-2 * a - b) + sin(-2 * (x * a) + b * x)/(-2 * a + b)))/4)

#Integrate(sin(x),x,a,b)
#Integrate(log(x),x,a,b)
#Integrate(1/(x^2-1),x)
a <- Sym("a")
b <- Sym("b")
Integrate(sin(a*x)^2*cos(b*x),x)

## yacas_expression((2 * sin(b * xs)/b - (sin(-2 * (xs * a) - b * xs)/(-2 * a - b) + sin(-2 * (xs * a) + b * xs)/(-2 * a + b)))/4)

Limits

#yacas("Limit(x,0)Sin(x)/x")
yacas("Limit(n,Infinity)(1+(1/n))^n")

## yacas_expression(exp(1))

yacas("Limit(h,0) (Sin(x+h)-Sin(x))/h")

## yacas_expression(cos(x))

#Limit(sin(x)/x,x,0)
ns <- Sym("ns")
Limit((1+(1/ns))^ns,ns,Infinity)

## yacas_expression(exp(1))

hs <- Sym("hs")
Limit((sin(xs+hs)-sin(xs))/hs,hs,0)

## yacas_expression(cos(xs))

Variable substitution

yacas("Subst(x,Cos(a))x+x")

## yacas_expression(2 * cos(a))

Subst(xs+xs,xs,cos(as))

## yacas_expression(2 * cos(as))

Solving ordinary differential equations

yacas("OdeSolve(y''==4*y)")

## yacas_expression(C545 * exp(2 * x) + C549 * exp(-2 * x))

yacas("OdeSolve(y'==8*y)")

## yacas_expression(C579 * exp(8 * x))

Inverse

yacas("E4i := Inverse(E4)")

## Yacas matrix:
##      [,1]                           [,2]                          
## [1,] u1 * u2^2/(u1^2 * u2^2)        1/u1 - u1 * u2^2/(u2^2 * u1^2)
## [2,] 1/u1 - u1 * u2^2/(u2^2 * u1^2) u1 * u2^2/(u2^2 * u1^2) - 1/u1
## [3,] -(u1 * u2/u1)/u2^2             u1 * u2/(u1 * u2^2)           
##      [,3]               
## [1,] -(u1 * u2/u1)/u2^2 
## [2,] u2 * u1/(u2^2 * u1)
## [3,] u1/u2^2

yacas("Simplify(E4i)")

## Yacas matrix:
##      [,1]  [,2] [,3]   
## [1,] 1/u1  0    -1/u2  
## [2,] 0     0    1/u2   
## [3,] -1/u2 1/u2 u1/u2^2

yacas("PrettyForm(Simplify(E4i))")

## 
## /                        \
## | / 1  \ ( 0 )  / -1 \   |
## | | -- |        | -- |   |
## | \ u1 /        \ u2 /   |
## |                        |
## | ( 0 )  ( 0 )  / 1  \   |
## |               | -- |   |
## |               \ u2 /   |
## |                        |
## | / -1 \ / 1  \ / u1  \  |
## | | -- | | -- | | --- |  |
## | \ u2 / \ u2 / |   2 |  |
## |               \ u2  /  |
## \                        /

E4i <- Inverse(E4)
Simplify(E4i)

## Yacas matrix:
##      [,1]  [,2] [,3]   
## [1,] 1/u1  0    -1/u2  
## [2,] 0     0    1/u2   
## [3,] -1/u2 1/u2 u1/u2^2

PrettyForm(Simplify(E4i))

## 
## /                        \
## | / 1  \ ( 0 )  / -1 \   |
## | | -- |        | -- |   |
## | \ u1 /        \ u2 /   |
## |                        |
## | ( 0 )  ( 0 )  / 1  \   |
## |               | -- |   |
## |               \ u2 /   |
## |                        |
## | / -1 \ / 1  \ / u1  \  |
## | | -- | | -- | | --- |  |
## | \ u2 / \ u2 / |   2 |  |
## |               \ u2  /  |
## \                        /

Determinant

yacas("Determinant(E4)")

## yacas_expression(-(u1 * u2^2))

yacas("Determinant(E4i)")

## yacas_expression((u1 * u2^2/(u2^2 * u1^2) - 1/u1) * (u1 * u2^2) * u1/(u1^2 * u2^4) - u1 * u2^2 * (u2 * u1) * (u1 * u2)/(u1 * u2^2 * (u2^2 * u1 * (u1^2 * u2^2))) - (1/u1 - u1 * u2^2/(u2^2 * u1^2)) * (u1 * u2)^2/(u2^4 * u1^2) - (1/u1 - u1 * u2^2/(u2^2 * u1^2))^2 * u1/u2^2 - (1/u1 - u1 * u2^2/(u2^2 * u1^2)) * (u2 * u1) * (u1 * u2)/(u2^4 * u1^2) - (u1 * u2^2/(u2^2 * u1^2) - 1/u1) * (u1 * u2)^2/(u1^2 * u2^4))

yacas("Simplify(E4i)")

## Yacas matrix:
##      [,1]  [,2] [,3]   
## [1,] 1/u1  0    -1/u2  
## [2,] 0     0    1/u2   
## [3,] -1/u2 1/u2 u1/u2^2

yacas("Simplify(Determinant(E4i))")

## yacas_expression(-1/(u1 * u2^2))

determinant(E4)

## yacas_expression(-(u1 * u2^2))

determinant(E4i)

## yacas_expression((u1 * u2^2/(u2^2 * u1^2) - 1/u1) * (u1 * u2^2) * u1/(u1^2 * u2^4) - u1 * u2^2 * (u2 * u1) * (u1 * u2)/(u1 * u2^2 * (u2^2 * u1 * (u1^2 * u2^2))) - (1/u1 - u1 * u2^2/(u2^2 * u1^2)) * (u1 * u2)^2/(u2^4 * u1^2) - (1/u1 - u1 * u2^2/(u2^2 * u1^2))^2 * u1/u2^2 - (1/u1 - u1 * u2^2/(u2^2 * u1^2)) * (u2 * u1) * (u1 * u2)/(u2^4 * u1^2) - (u1 * u2^2/(u2^2 * u1^2) - 1/u1) * (u1 * u2)^2/(u1^2 * u2^4))

Simplify(E4i)

## Yacas matrix:
##      [,1]  [,2] [,3]   
## [1,] 1/u1  0    -1/u2  
## [2,] 0     0    1/u2   
## [3,] -1/u2 1/u2 u1/u2^2

Simplify(determinant(E4i))

## yacas_expression(-1/(u1 * u2^2))