## Visualizing Complex Functions with Conformal Mapping

For the thoroughly modern calculus student, an introduction to Complex Variables is all the more daunting because we don’t have the kind of geometric intuition-building machinery available to us for functions $f: C \rightarrow C$ as we did for real-valued functions in calculus.  Not that attempts haven’t been made….  In the coming weeks, I want to work on several techniques in Maxima.  Here’s a first approach:  conformal maps.  The code drawgridC below is limited in its versatility, but couldn’t be simpler to call.  Here are some examples,

/* plot the traces of complex valued function _f
along grid lines in the square
-1 <= Real(z) <= 1, -1<=Ima(z)<=1
blue - images of _f along horizontal lines
red - images of _f along vertical lines */
drawgridC(_f):=block(
[fx,fy,fxt,fyt,pl,j,ngrid],
fx:realpart(rectform(subst(z=x+%i*y,_f))),
fy:imagpart(rectform(subst(z=x+%i*y,_f))),
pl:[],
ngrid:19,
for j:0 thru ngrid do(
fxt:subst([x=-1+j*2/ngrid,y=t],fx),
fyt:subst([x=-1+j*2/ngrid,y=t],fy),
pl:cons([parametric(fxt,fyt,t,-1,1)] , pl)
),
pl:cons([color=red],pl),
for j:0 thru ngrid do(
fxt:subst([y=-1+j*2/ngrid,x=t],fx),
fyt:subst([y=-1+j*2/ngrid,x=t],fy),
pl:cons([parametric(fxt,fyt,t,-1,1)] , pl)
),
pl:cons([color=blue],pl),
pl:cons([xlabel="",ylabel=""],pl),
pl:cons([nticks=200],pl),
apply(wxdraw2d,pl)
);

## Complex Factors of Real and Complex Polynomials – part 2

Last week I posted a Maxima utility to factor polynomials using complex roots.  The idea was to use the built-in solver solve to find each root $a$ with multiplicity $m$, and then construct a factor of the form $(z-a)^m$.  Typically, solve can fail to identify some roots that are returned by the more robust to_poly_solve.  My problem at that time was that I didn’t know how to access the multiplicities for the roots returned by to_poly_solve.

I’ve written a utility to determine multiplicities for the roots returned by to_poly_solve. With that, here’s an updated version of factorC that takes advantage of the more robust solver.

/* factor a polynomial using all its complex roots */
factorC(_f,_z):=block(
[s,n,m,fp,j],
fp:1,
ss:to_poly_solve(_f,_z),
s:create_list(args(ss)[k][1],k,1,length(ss)),
m:tpsmult1(_f,_z,ss),
/*These lines were needed before I figured out how
how to handle multiplicities with to_poly_solve
s:solve(_f,_z),
m:multiplicities,*/
n:length(s),
for j:1 thru n do
if lhs(s[j])#0
then fp:fp*(_z-(rhs(s[j])))^m[j],
fp:fp*divide(_f,fp)[1],
fp
);

## Roots, Multiplicities, and to_poly_solve

A few days ago I posted some code to factor and perform partial fraction decomposition using all available complex roots.  The approach is to find roots $a$ — taking note of the multiplicity $m$ of each —  and then construct factors of the form $(z-a)^m$.

As I’ve noted before, the built-in command solve can fail to find solutions.  The more robust Maxima solver is to_poly_solve.    That said, after a call to to_poly_solve, I don’t know how to get my hands on the multiplicities.  So, I’ve written a little utility to compute and return a list of multiplicities for the roots returned by to_poly_solve.  Here’s an example, followed by the code.  Notice that because to_poly_solve returns solutions using the %union construction, extracting the $j$th root requires something like root:rhs(part(s,j,1))

/* tpsmult1 assumes results of to_poly_solve applied to _f,
are passed in the argument s */
tpsmult1(_f,_z,s):=block(
[d,root,nroots,mult,m1],
nroots:length(s),
mult:[],
for j:1 thru nroots do (
root:rhs(part(s,j,1)),
m1:rootmult(_f,_z,root),
mult: endcons(m1,mult)
),
mult
);

/* find the multiplicity of a root _a for the polynomial _f */
rootmult(_f,_z,_a):=block(
[d,val,m,mm],
m:1,
d:divide(_f,(_z-_a)),
val:subst(_z=_a,part(d,1)),
if (val=0) then (
mm:rootmult(part(d,1),_z,_a),
m:m+mm
),
m
);

## Partial Fraction Decomposition Using Complex Factors

A few days ago I wrote about my solution for factoring a polynomial into factors corresponding to complex roots,  factorC.

One of the uses for that utility is to perform a partial fractions decomposition of rational functions.  Here’s a complex partial fraction function that uses factorC and the built-in Maxima function partfrac.

partfracC(_f,_z):=block(
[d,fd],
d:denom(_f),
fd:factorC(d,_z),
partfrac(1/fd,_z)
);

## Complex Factors of Real and Complex Polynomials

I’m getting ready for my fall Complex Variables class.  I noticed that the built-in Maxima function residue doesn’t reliably do the right thing.  My goal is to make some improvements to  Maxima residue calculations in Maxima over the course of the next month.

As I started to look at some test cases, I realized I didn’t know how to factor a polynomial into complex factors.  In the simplest case:

but I wanted to see

Maybe someone will find this and let me know that there’s a simple way to make that happen using existing Maxima commands.  Until then, I’ve written a little utility  to identify the roots (both real and complex) of a polynomial and return a factorization.  Here’s an example, and the code.  Notice that it only works as well as the root finder solve.  I tried to upgrade to the more robust to_poly_solve,  but I don’t yet know how to handle multiplicities in that case.

factorC(_f,_z):=block(
[s,n,m,fp,j],
fp:1,
/* This commented code was meant to use the
more robust solver to_poly_solve, but
I couldn't understand how to handle multiplicities
ss:args(to_poly_solve(_f,_z)),
s:create_list(ss[k][1],k,1,length(ss)),*/
s:solve(_f,_z),
m:multiplicities,
n:length(s),
for j:1 thru n do
if lhs(s[j])#0
then fp:fp*(_z-(rhs(s[j])))^m[j],
fp:fp*divide(_f,fp)[1],
fp
);