## Collecting all my little matlab-like Maxima widgets in one place

Since I’ve been using Maxima (circa 2016), I’ve occasionally missed some little feature from matlab and coded up a replacement for maxima, with a corresponding blog piece here at the Maximalist.

Some examples are find(), diff(), pause(), size(), cumsum(), diag(), and a few list indexing utilities. Also a help() utility that mimics matlab.

Here’s a mac file with all of those in one easy-to-load place: matlabesque.mac

Its help() entry reads like this:

``````matlabesque.mac contains:
find(exp)
ithruj(L,i,j)
indexby(L,indexlist)
matlab_diff(L)
pause([options])
cumsum(l)
size(M)
diag(M)
-
-
for any of the above functions,
help(function_name) returns help lines for function_name``````

## C’est une pipe for Maxima

I really love the pipe operator \$>\$ introduced into R by the package magrittr.  Not least because of the reference to the piece pictured here — “The Treachery of Images” by the Belgian surrealist painter René Magritte.

I don’t know yet whether this will be a really useful Maxima feature, but it is easy enough to make a simple pipe operation using infix():

```infix("%>%",1,1);
"%>%" (a,b):=b(a);```

Notice the 2nd and 3rd arguments of infix() control the precedence of operations (the left and right hand binding powers lbp and rbp).  We want to set those values lower than lbp and rbp for multiplication * in 2*%pi  and for for addition + in 1+%i (which by default in Maxima have lbp=180 and rbp=180).  Here’s the maxima document that gives the details about operators and binding power.

To make the pipe idea slightly more powerful so as to work with functions like integrate() and diff() which require more than one argument, we could try something like this, which makes use of the matlab-like find() I wrote about recently.

```infix("%>%",1,1);
"%>%" (a,b):=block([i,bb],
if atom(b) then
b(a)
else (
i:find(b=%P),
bb:append(firstn(b,i[1]-1),[a],rest(b,i[1])),
apply(first(bb),rest(bb,1))
)
);```

Note that we’ve introduced the convention that the piped expression is referred to as %P, and that we call functions using a lisp-like construction where diff(%P,x) becomes [diff,%P,x].

## Two little list utilities for MATLAB-like array indexing

I grew up with MATLAB, where extracting a subset of a vector V was easy as feeding a vector of indices into a vector.  For example entries i thru j could be had with V(i:j) and an more complicated index scheme could be accomplished with a vector of indices i_index and then V(i_index).

In Maxima, first(), last(), firstn(), rest(), and most generally makelist() allows for all that and more but with a little more cumbersome calling protocols.  Here are one-liners that achieve something like the two canonical MATLAB examples above:

## A maxima function to replicate MATLAB diff() and an efficiency comparison

I needed a function to compute a list containing the difference between adjacent entries in another list, as diff() does for an array in MATLAB.

I first wrote  something using makelist() and found it very slow for large lists.  I then rewrote the function using rest() and found it much faster:

## Surface Integrals, Triple Integrals, and the Divergence Theorem of Gauss in Maxima

In earlier posts, I describe the Package of Maxima functions MATH214 for use in my multivariable calculus class, with applications to Greens  Theorem and Stokes Theorem.

Here we show how the  surface integral function integrateSurf() and triple integration  function integrate3()  (together with the divergence function div() )work on a Gauss’s Theorem example:

We integrate the parabolic surface and the circular base surface separately, and show their sum is equal to the triple integral of the divergence.

The functions above are included in the MATH214 package, but I list them below as well:

```integrateSurf(F,S,uu,aa,bb,vv,cc,dd):=block(
[F2],
F2:psubst([x=S[1],y=S[2],z=S[3]],F),
integrate(integrate(trigsimp(F2.cross(diff(S,uu),diff(S,vv))),uu,aa,bb),vv,cc,dd));

integrate3(F,xx,aa,bb,yy,cc,dd,zz,ee,ff):=block(
integrate(integrate(integrate(F,xx,aa,bb),yy,cc,dd),zz,ee,ff));

div(f,x,y,z):=diff(f[1],x)+diff(f[2],y)+diff(f[3],z)\$

cross(_u,_v):=[_u[2]*_v[3]-_u[3]*_v[2],_u[3]*_v[1]-_u[1]*_v[3],_u[1]*_v[2]-_u[2]*_v[1]]\$```

## Path Integrals in the Plane, Double Integrals, and Greens Theorem in Maxima

In an earlier post I described the Maxima package MATH214 for use in my multivariable calculus class.  I’ve posted examples with applications to Gauss’s  Theorem and Stokes Theorem.

Here we take the double integration routine integrate2() and the 2D path integral integratePathv2() for a spin with a Green’s Theorem example from Stewart’s Calculus Concepts and Contexts:

And of course polar coordinates are nice too:

The two functions used above are included in the MATH214 package, but I list them below as well.

```integratePathv2(H,r,t,a,b):=block(
[H2],
H2:psubst([x=r[1],y=r[2]],H),
integrate( trigsimp(H2.diff(r,t)),t,a,b)
);

integrate2(F,xx,aa,bb,yy,cc,dd):=block(
integrate(integrate(F,xx,aa,bb),yy,cc,dd));```

## Surface Integrals and Stokes Theorem in Maxima

In an earlier post I detailed the Maxima functions contained  the MATH214 package for use in my multivariable calculus class.  The package at that link has now been updated with some further integration utilities:  integrate2() and integrate3() for double and triple integrals and integrateSurf() for surface integrals of vector fields in 3D.  I’ve posted examples with applications to Green’s  Theorem and Gauss’s Theorem.

Here’s a test drive of the surface integration function using a Stokes theorem example I found on the web:

Verify Stokes theorem for the surface S described by the paraboloid z=16-x^2-y^2 for z>=0

and the vector field

F =3yi+4zj-6xk

First the path integral of the vector field around the circular boundary of the surface using integratePathv3() from the MATH214 package

And also the surface integral using integrateSurf().  Notice that in the order of integration we specify in integrateSurf(),  (first  y then x) the surface normal vector computed with cross() in that same variable order points inward—the negative orientation.  We reverse the direction with an extra negative inside the surface integral.

Although they are included in the MATH214 package, here are the  functions used above:

```integrateSurf(F,S,uu,aa,bb,vv,cc,dd):=block(
[F2],
F2:psubst([x=S[1],y=S[2],z=S[3]],F),
integrate(integrate(trigsimp(F2.cross(diff(S,uu),diff(S,vv))),uu,aa,bb),vv,cc,dd));

cross(_u,_v):=[_u[2]*_v[3]-_u[3]*_v[2],_u[3]*_v[1]-_u[1]*_v[3],_u[1]*_v[2]-_u[2]*_v[1]]\$

curl(f,x,y,z):=[ diff(f[3],y)-diff(f[2],z),curl(f,x,y,z):=[ diff(f[3],y)-diff(f[2],z),                     diff(f[1],z)-diff(f[3],x),   diff(f[2],x)-diff(f[1],y) ]\$
integratePathv3(H,r,t,a,b):=block(
[H3],
H3:psubst([x=r[1],y=r[2],z=r[3]],H),
integrate( trigsimp(H3.diff(r,t)),t,a,b)
);```

## An improved Maxima function for inverse Laplace transform

Maxima has a fairly serviceable Laplace transform utility built-in.  Here’s an example from the popular ordinary differential equations book by Blanchard, Devaney and Hall:

Trouble arises when we look at discontinuous forcing functions, which is especially sad because it seems to me that’s what makes it worthwhile to spend time with Laplace transforms.  In particular, the inverse transform function ilt() fails on the Heaviside Function and Dirac Delta, even though the built-in laplace() treats them correctly:

So, I’ve written an alternative inverse Laplace function laplaceInv() that fixes that problem:

Here are a few differential equation solutions to show how the new function behaves:

A second-order linear equation with a constant forcing function that vanishes at $t=7$

A second-order equation with two impulsive forces: