## Path Integrals

For my multivariable calculus class, I wanted an easy-to-call suite of symbolic integrators for path integrals of the form

$\int_C f(x,y) dr$,

$\int_C {\bf F}(x,y)\cdot d{\bf r} = \int_C \langle P(x,y), Q(x,y) \rangle\cdot \langle dx, dy \rangle$, or

$\int_C {\bf F}(x,y,z)\cdot d{\bf r} = \int_C \langle P(x,y,z), Q(x,y,z), R(x,y,z) \rangle\cdot \langle dx, dy, dz \rangle$.

My overarching design idea was that the input arguments needed to look the way they do when I teach the course:

• a scalar field $f(x,y):R^2 \rightarrow R$ or a vector field  ${\bf F}(x,y): R^2 \rightarrow R^2$ or ${\bf F}(x,y,z): R^3 \rightarrow R^3$
• a curve $C$ defined by a vector-valued function ${\bf r}(t): R \rightarrow R^n, a\le t \le b$ where $n=2,3$ as appropriate.

It took me a while to work out how to evaluate the integrand along the path within my function.  Things that worked fine on the command line failed when embedded into a batch file to which I passed functions as arguments.  I ended up using subst, one variable at a time.  I’d like to be able to do this in a single command which can detect whether we’re in 2 or 3 dimensions so that I don’t need separate commands.

For now, here’s what I came up with along with some illustrative examples taken from Paul’s online math notes, that show how to call these new commands I, I2 and I3.

/* path integral of a scalar integrand f(x,y) on path r(t) in R^2, t from a to b */
I(f,r,t,a,b):=block(
[f1,f2,dr,Iout],
f1:subst(x=r[1],f),
f2:subst(y=r[2],f1),
dr:sqrt(diff(r,t).diff(r,t)),
Iout: integrate(f2*dr,t,a,b),
Iout
);
/* path integral of a vector integrand F(x,y) on path r(t) in R^2, t from a to b */
I2(H,r,t,a,b):=block(
[H1,H2,I],
H1:subst(x=r[1],H),
H2:subst(y=r[2],H1),
I: integrate( H2.diff(r,t),t,a,b),
I
);
/* path integral of a vector integrand F(x,y,z) on path r(t) in R^3, t from a to b */
I3(H,r,t,a,b):=block(
[H1,H2,H3,I],
H1:subst(x=r[1],H),
H2:subst(y=r[2],H1),
H3:subst(z=r[3],H2),
I: integrate( H3.diff(r,t),t,a,b),
I
);

Here’s an update:  a related maxima function for evaluating a complex integral

$\int_\Gamma f(z) dz$

where $f: C \rightarrow C$ and the curve $\Gamma$ is given by $r: R \rightarrow C$.

/* path integral of a complex integrand f(z): C --> C, on path z(t): R --> C, t from a to b */
IC(f,r,t,a,b):=block(
[f1,dz,Iout],
f1:subst(z=r,f),
dz:diff(r,t),
Iout: integrate(f1*dz,t,a,b),
Iout
);

## Two Maxima Functions for Riemann Sums

Two early attempts at programming Maxima functions.  I’d love to hear your comments about how to make these work better.  Many thanks to the online community from whom I learned how to get this far!

The script is given at the bottom of this post.  Copy and paste into a file (in your working directory) called Riemann.mac, and then load into Maxima using

/* Two Maxima functions for introductory integral calculus
(c) 2016, themaximalist.org  */

/* RiemannSum
fn is the function to be integrated on the interval [a,b]
n rectangles
opt specifies:
0: left endpoints
1: midpoints
2: right endpoints
*/

RiemannSum(fn,a,b,n,opt):=
block([xx,s],
xx(i,n):= a+(i+opt/2)*(b-a)/n,
s: sum(ev(fn,x=xx(i,n)),i,0,n-1)*(b-a)/n,
float(s)

)$/* RiemannRectangles to Draw rectangles fn is the function to be integrated: on the interval [a,b] n rectangles opt specifies: 0: left endpoints 1: midpoints 2: right endpoints */ load(draw)% RiemannRectangles(fn,a,b,n,opt):= block([xi,xx,rects,wxd], xi(i,n):= a+i*(b-a)/n, xx(i,n):= a+(i+opt/2)*(b-a)/n, rects(n):=makelist(rectangle([xi(k,n),0], [xi(k+1,n),ev(fn,x=xx(k,n))]),k,0,n-1), wxd(n):=apply(wxdraw2d, append([xrange=[a-(b-a)/4, b+(b-a)/4],color=blue], rects(n), [transparent=true, explicit(fn,x,a, b)] ) ), wxd(n) )$