## Walk on Spheres Method in Julia

I saw a cool tweet (and corresponding conference paper) by Keenan Crane

http://www.cs.cmu.edu/~kmcrane/Projects/MonteCarloGeometryProcessing/index.html

I was vaguely aware that one can use a Monte Carlo method to solve the boundary value Laplace equation $\nabla^2 \phi = 0$ , but I don’t think I had seen the walk on spheres variant of it before. I think Crane’s point is how similar all this is to stuff graphics people already do and do well. It’s a super cool paper. Check it out.

Conceptually, I think it is plausible that the Laplace equation and a monte carlo walk are related because the static diffusion equation $\nabla^2 n = 0$ from Fick’s law ultimately comes from the brownian motion of little guys wobbling about from a microscopic perspective.

Slightly more abstractly, both linear differential equations and random walks can be describe by matrices, a finite difference matrix (for concreteness) K and a transition matrix of jump probabilities T. The differential equation is discretized to Kx=b and the stationary probability distribution is Tp=b, where b are sources and sinks at the boundary.

The mean value property of the Laplace equation allows one to speed this process up. Instead of having a ton of little walks, you can just walk out randomly sampling on the surface of big spheres. en.wikipedia.org/wiki/Walk-on-spheres_method. Alternatively you can think of it as eventually every random walk exits a sphere, and it is at a random spot on it.

So here’s the procedure. Pick a point you want the value of $\phi$ at. Make the biggest sphere you can that stays in the domain. Pick a random point on the sphere. If that point is on the boundary, record that boundary value, otherwise iterate. Do this many many times, then the average value of the boundaries you recorded it the value of $\phi$

This seems like a good example for Julia use. It would be somewhat difficult to code this up efficiently in python using vectorized numpy primitives. Maybe in the future we could try parallelize or do this on the GPU? Monte carlo methods like these are quite parallelizable.

The solution of the 1-d Laplace equation is absolutely trivial. If the second derivative is 0, then . This line is found by fitting it to the two endpoint values.

So we’re gonna get a line out

using LinearAlgebra
avg = 0
phi0 = 0
phi1 = 10
x_0 = 0.75
function monte_run(x)
while true
l = rand(Bool) # go left?
if (l && x <= 0.5) # finish at left edge 0
return phi0
elseif (!l && x >= 0.5) # finish at right edge 1
return phi1
else
if x <= 0.5 # move away from 0
x += x
else
x -= 1 - x # move away from 1
end
end
end
end

monte_runs = [monte_run(x) for run_num =1:100, x=0:0.05:1 ]
import Statistics
avgs = vec(Statistics.mean( monte_runs , dims=1))
stddevs = vec(Statistics.std(monte_runs, dims=1)) ./ sqrt(size(monte_runs)) # something like this right?

plot(0:0.05:1, avgs, yerror=stddevs)
plot!(0:0.05:1,  (0:0.05:1) * 10 )

And indeed we do.

You can do a very similar thing in 2d. Here I use the boundary values on a disc corresponding to x^2 – y^2 (which is a simple exact solution of the Laplace equation).



function monte_run_2d(phi_b, x)
while true
r = norm(x)
if r > 0.95 # good enough
return phi_b(x)
else
dr = 1.0 - r #assuming big radius of 1
θ = 2 * pi * rand(Float64) #
x += dr * cos(θ)
x += dr * sin(θ)
end
end
end

monte_run_2d( x -> x,  [0.0 0.0] )

monte_runs = [monte_run_2d(x -> x^2 - x^2 ,  [x 0.0] ) for run_num =1:1000, x=0:0.05:1 ]

import Statistics
avgs = vec(Statistics.mean( monte_runs , dims=1))
stddevs = vec(Statistics.std(monte_runs, dims=1)) ./ sqrt(size(monte_runs)) # something like this right?
plot(0:0.05:1, avgs, yerror=stddevs)
plot!(0:0.05:1,  (0:0.05:1) .^2 )


There’s more notes and derivations in my notebook here https://github.com/philzook58/thoughtbooks/blob/master/monte_carlo_integrate.ipynb

## A Smattering of Physics in Sympy

Sympy is fun. I’ve been enjoying trying out some simple physics problems and seeing what kind of fun angles sympy brings to the table. It does pretty good on concrete problems, not so good at abstract derivations.

### Kinematics

Ah such fond memories! In high school, I was taught by Ric Thompson “the big four”. $x_f = x_i + v_i t + \frac{1}{2} a t^2$ $v_f = v_i + a t$ $v_f^2 = v_i^2 + 2 a d$ $d = \frac{v_f + v_i}{2} t$

The equations are of course, overcomplete. They are all implied by $\frac{d^2}{dt^2}x = a$, but even with only algebra then second two are derivable from the first two.

Of course a natural way of deriving the equations is to solve one equation for a variable and substitute it into the other equation. sympy makes this pretty dang easy.

from sympy import *
init_printing()
t,a,d,vf,vi = symbols("t a d vf vi")
e1 = Eq(d , vi * t + 1/2 * a * t ** 2)
tsub = solve(Eq(vf , vi + a * t),t)
print(tsub) # This is assuming a is nonzero though.
expand(simplify(e1.subs(t,tsub)))
(vf - vi)/a
Eq(d, 0.5*vf**2/a - 0.5*vi**2/a)

However, there is a more automated approach.

It turns out that a decent chunk of physics equations are or can be well approximated by a system of polynomial equations. There are systematic methods that are guaranteed to solve the problem (albeit maybe not in the lifetime of the universe).

A grobner basis is an equivalent set of polynomial equations that has useful properties. For some simple purposes, all you need to know is that if you give the variables you want to eliminate first, the Groebner basis will contain equations without those variable. Here we specify t as one to eliminate, so we get an equation without t in it


G = groebner(  [vi * t + 1/2 * a * t ** 2 - d,
vi + a * t - vf] ,
t,vf,d,a,vi  )
for e in G:
print(e)
-2.0*d + 1.0*t*vf + 1.0*t*vi
1.0*a*t - 1.0*vf + 1.0*vi
-2.0*a*d + 1.0*vf**2 - 1.0*vi**2

I’ve actually been pleasantly surprised at how many physics problems reduce ultimately to systems of polynomial constraints. Energy and momentum conservation are polynomial constraints (classical feynman diagrams kind of). Special relativity questions can be reduced to polynomial constraints using the proper time.

#elephant problem
# elephants give birth at 21 months. On a rocket at velocity v
# how long T until you see it give birth?
tau , t1, t2, x1, v, c, T = symbols("tau t1 t2 c1 v c T")

eqs = [
tau**2 - (t1**2 - x1**2 / c**2), # proper time
x1 - v * t1, # distance away
c * t2 - x1, # time for light to travel back
T - t1 - t2, # total time
tau - 21 # proper time is 21 months

]

groebner(eqs, tau , t1, t2, x1, v, T)

### Lagrangian Mechanics

The Structure and Interpretation of Classical Mechanics is an interesting book.

It points out that notation we use is extremely imprecise and implicit. This is a source of great confusion.

A great benefit of programming up such examples is that it makes explicit (sometimes painfully so) steps that were implicit before.

In the Euler Lagrange equation, first partially differentiates considering q and $\dot{q}$ to be independent parameters. Then a substitution is makde for a function $x(t)$ and then we procede with a differentiation with respect to time.

# simple harmonic oscillator lagrangian style
m, k = symbols("m k", real = True, positive=True)
v, q = symbols("v q")
K = Rational(1,2) * m * v ** 2 #kinetic energy
V = Rational(1,2) * k * q ** 2 # potential energy
L =  K - V  #Lagrangian
F = diff(L,q) # force
p = diff(L,v) # momentum

x_ = Function("x")
t = symbols("t")

x = x_(t)

subst = { v : diff(x,t),
q : x} # replacement to turn q into a function x

# euler-lagrange equations of motion
eq = F.subs( subst ) - diff( p.subs(subst)  , t )
dsolve(eq) # general solution cosine and sines

Here’s an analogous thing for a pendulum

#simple harmonic oscillator lagrangian style
m, g, L = symbols("m g L", real = True, positive=True)
theta, w = symbols("theta omega")
K = Rational(1,2) * m * (w * L) ** 2 #kinetic energy
V = - Rational(1,2) * m * g * L * cos(theta) # potential energy. angle is defined as 0 = hanging down
L =  K - V  #Lagrangian
F = diff(L,theta) # force
p = diff(L,w) # momentum
F
p

x_ = Function("theta")
t = symbols("t")

x = x_(t)

subst = { w : diff(x,t),
theta : x} # replacement to turn q into a function x

# euler-lagrange equations of motion
eq = F.subs( subst ) - diff( p.subs(subst)  , t )
eq
#dsolve(eq) 

Another place where an implicit stated substitution is absolutely vital is in the Legendre transform going from the Lagrangian to the Hamiltonian.

# legendre transformation to hamiltonian
p = symbols( "p" )
H_ = p * v - L # hamiltonian but we haven't solved out v yet
v_of_pq = solve(diff(H_, v), v) # set derivative to 0 to solve for v.
H = simplify(H_.subs(v, v_of_pq )) # substitue back in. Here is the actual hamiltonian
H

#### Statistical Mechanics

Sympy can do Gaussian integrals! How convenient. It can also do power series expansions. And differentiate. So it takes the drudgery out of some simple calculations

# ideal gas partition function
beta, m, V, N, kb, T  = symbols("beta m V N k_b T", real=True, positive=True)
p = symbols("p", real=True)
Z = integrate( exp( - beta * Rational(1,2) * p ** 2 / m ), (p,-oo,oo))**(3*N) * V**N #partition function
def avg_energy(Z):
return - diff(ln(Z), beta).subs(beta, 1/ kb / T)
print(avg_energy(Z)) #
F = (-ln(Z) / beta).subs(beta, 1 / kb / T) #helmholtz free energy
S = diff(F , T) # sentropy is derivative of helmholtz wrt T
S # the functional dependence on T and V I think is correct
P = -diff(F , V) # pressure is - derivative of V
P
# Neato
# hamrmonic oscillator partition function
beta, m, k = symbols("beta m k ", real=True, positive=True)
p, x = symbols("p x", real=True)
E = R(1,2) * p ** 2 / m  + R(1,2) * k * x ** 2
Z = integrate( integrate( exp( - beta * E ), (p,-oo,oo)) , (x,-oo, oo))**N
diff(-ln(Z),beta)

Perturbation theory of the partition function of an anharmonic oscillator. Pretty easy. It is interesting to note that this is the very simplest schematic of how perturbation theory can be approached for quantum field theory.

# pertubration theory of anharmonic oscillator
beta, m, k, g = symbols("beta m k g ", real=True, positive=True)
p, x = symbols("p x", real=True)
E = Rational(1,2) * ( p ** 2 / m  +  k * x ** 2) + g * x ** 4
series(exp( - beta * E ), g).removeO()
Z = integrate( integrate( series(exp( - beta * E ), g, n=2).removeO(), (p,-oo,oo)) , (x,-oo, oo))
simplify(diff(-ln(Z),beta)) #E
simplify(diff(-ln(Z),k)/beta) #<x**> 

Other things that might be interesing : 2 oscillators, A chain of oscillators, virial expansion

#### Thermo and Legendre Tranformations

Thermodynamics is a poorly communicated topic. Which variables remain in expressions and what things are held constant when differentiating are crucial and yet poorly communicated and the notation is trash. Sympy helps make some things explicit. It’s fun.

u,s,t,p,v,n,r = symbols("u s t p v n r")

du,ds,dt,dp,dv = symbols("du ds dt dp dv")
# taylor series in stuff?

e1 = p * v - n * r * t
e2 = u - Rational(3 , 2) * n * r * t

state = [  (u,du), (s,ds), (t,dt) , (p,dp) , (v,dv) ]

def differential(e):
return sum( [ diff(e,x) * dx  for x,dx in state]   )

de1 = differential(e1 )
de2 = differential(e2 )

e3 = du - (t * ds - p * dv)

eqs = [e1,e2,de1,de2,e3]
print(eqs)
G = groebner( eqs, u , du,  t, dt, p, dp, v, dv,  ds )
for e in G:
print(e)
R = Rational
U,S,T,P,V,N, k = symbols("U S T P V N k")

cv = R(3,2) * N * k
e1 = U - cv * T
e2 = P * V - N * k * T
e3 = S - cv * ln(T) + N * k * ln(V)

elim = [P,T]
Ps = solve([e1,e2,e3], P)
Ps
es = [ e.subs(Ps) for e in [e1,e2,e3] ]
Ts = solve(e3, T)
es = [  e.subs(T,Ts) for e in es ]
Usv = solve(es,U)
psv = diff(Usv,V)
tsv = diff( Usv , S )

#solve(es, V)

Hsv = Usv + P * V  # enthalpy and legendre trnasformation
Vps = solve(diff(Hsv, V) , V)
H =  Hsv.subs(V, Vps)
simplify(H)

There are so many other things!

What about a Van Der Waals equation? Optics (geometrical and wave, paraxial ~ Schrodinger, fourier optics), GR (exterior derivatives ) , Quantum (wave matching problems. What can we do about hydrogen? WKB, QHE) rutherford scattering, Weiss mean field, canonical transformations, Rotations. Clebsh-Gordon coefficients

## Computational Category Theory in Python III: Monoids, Groups, and Preorders

Parts 1 and 2 are found here and here

From one perspective, categories are just another algebraic structure, like groups, monoids and rings. They are these abstract things that have some abstract equational axioms and operations. They are the next stop on our magnificent category journey.

A monoid is a thing that has an associative operation with a unit. Addition and 0 make numbers a monoid. Multiplication and 1 are a separate monoid for numbers. Concatenation and empty lists make lists a monoid. Union and empty set make sets a monoid. We can encode this in python like so:

What does this have to do with categories? Well, if some thing is a category, it obeys the axioms that define what it means to be a category. It has morphisms and objects. The morphisms compose if head meets tail on an object. There are always identity morphism.

The morphisms in a category with 1 object automatically obey the monoid axioms. In this case, the category axioms imply the monoid axioms. Everything composes because there is only one object. It’s a kind of degenerate case where we are not using the partiality of the composition operator. There is automatically a unit for composition because the identity morphism is a unit. Composition is already required to be associative. Boom. The thing is a monoid.

Continuing with our representation from previous posts, we make a python class for each category. An instance of this class is a morphism in this category. If you ask for the domain or codomain of any morphism, you always get back () because it is a single object category. Compare these classes with the above classes.

Some monoids are also groups if there is a natural inverse operation. The integers are a group under addition where the negative gives you the inverse. Some aren’t though. The natural numbers (0,1,2…) aren’t a group under addition though.

Similarly groups can be thought of as a category with one object, with the additional requirement that every morphism is invertible, that there is always a $f^{-1}$ such that $f \circ f^{-1} = id$.

Sympy has groups in it. We can make a wrapper of that functionality that looks like a categorical interface. To match our pattern of using python classes to represent categories, it is convenient to do the slightly uncommon thing of making a class definition generator function fp_group_cat. Every time you call this function, it makes a different class and a different category. I have only here wrapped the finitely presented group functionality, but there are also free groups, permutation groups, and named groups available in sympy.

#### Many objects, at most one arrow per pair: Preorders

We can simplify the power of a category in a different direction. Instead of having only 1 object, we’ll have few arrows.

A category with many objects but at most a single morphism between a pair of them obeys the axioms of a preorder. In categorical terminology this is sometimes called a thin category Any actual order like like $\le$ on numbers is also a preorder, but preorders have slightly weaker requirements. Here is a categorical representation of the ordering on integers (although really the same implementation will work for any python type that implements <= and == )

An example of a partial order is the subset relationship, which we can represent using python sets. This is an important but perhaps confusing example. Haven’t we already defined FinSet? Yes, but these are different categories. In FinSet, morphisms are functions. In SubSetCat a morphisms is the subset relationship (of which there can either be one or not one). They just plain are not the same thing even though there are sets in the mix for both. The situation is made even more confusing by the fact that the subset relationship can be talked about indirectly inside FinSet using monic morphisms, which have as their image the subset of interest.

Preorders are related to directed acyclic graphs (DAG), the directed graphs that have no loops. If you give me a DAG, there is a preorder that is generated by that DAG. Exercise for the reader (AKA I’m lazy): Can you turn a Networkx DAG into a category?

### Thoughts

This is nice and all just to explain categories in terms of some perhaps more familiar concepts. It feels a little ho-hum to me. We are not getting really any benefit from the concept of a category from this post. However, the examples of monoids, groups and preorders are always something you should think about when presented when a new categorical concept, because it probably reduces to something more familiar in this case. In addition, mappings to/from these simple objects to more complicated categories can be very interesting.

The methods of computational group theory are intriguing. It seems like some of them should extend to category theory. See this book by RFC Walters for example https://www.cambridge.org/core/books/categories-and-computer-science/203EBBEE29BEADB035C9DD80191E67B1 A very interesting book in other ways too. (Thanks to Evan Patterson for the tip)

Next time I think we’ll talk about finite categories and the finite Yoneda lemma.

Artwork courtesy of David

Edit: Hacker News discussion: https://news.ycombinator.com/item?id=23058551