# Configuration integral for a variable number of points on a torus

If I have $n$ points on the surface of a torus, and want to check the Euclidean length of all "three-hop paths" between two (newly added) fixed points $s,t$ at distance $||s-t||$ apart, I need to check the sum of the edge lengths over $2 {n \choose 2}$ paths.

With $n=3$ there are 6 paths I can form. With $n=4$, there are 12.

Using Boole, I can determine if any of these path lengths are shorter than some critical value, e.g. for $(x,y)$ and $(X,Y)$ two points in the plane....

Boole[Sqrt[X^2 + Y^2] + Sqrt[(X - x)^2 + (Y - y)^2] +
Sqrt[(d - x)^2 + y^2] < L]


returns 1 whenever the three-hop path is shorter than a length $L$, with $||s-t||=d$, and zero otherwise.

With three points, as mentioned, there are six routes. I would need six instances of Boole like the one above, each in 4 variables, in order to check all the paths to see if any were shorter than $L$.

I could then integrate these functions over all configurations of 3 points, to see what proportion of all configurations have a path of length less than $L$, for example.

Is there a way to write a function which takes in $n$, a natural number, and returns the Boole as above for me, with each of the lengths used in inclusive OR? So the above equation would be returned when $n=2$. With $n=3$, it would have $6$ separate conditions, any one of which returning true gives this Boole function the value 1, and is otherwise zero.

• – apkg
Jan 31 '18 at 15:53
• Yes, also the paths a three “hops” long
– apkg
Jan 31 '18 at 20:09
• I need to set up a numerical integral which gives the CDF of the length of the shortest path s —> t via two other nodes
– apkg
Jan 31 '18 at 20:10
• So, if I understand correctly, given $n$ points, you'd like a function fun[s, t, L] that would return the proportion of 3-hop paths that are shorter than $L$ among all 3-hop paths. Then, you would integrate fun over s and t. This brute-force approach might be time-consuming... Jan 31 '18 at 20:23
• Yes. But is it possible to do? If it worked only for small n that would be fine
– apkg
Jan 31 '18 at 20:25

The following function ratio takes two points s and t, the list of $n$ points pts, and it returns the ratio of lengths shorter than length. It runs in 2ms for n=50 which is reasonable.

n = 50;
length = 4;
pts = RandomReal[{-1, 1}, {n, 2}];
s = {-1, 1}; t = -s;

ratio[s_, t_, l_] := Block[{},
paths = {s}~Join~#~Join~{t} & /@ Subsets[pts, {2}];
lengths = Total[Norm /@ Differences[#]] & /@ paths;
1 - .5*Total[Sign[lengths - l] + 1]/Length@lengths]


To be used as ratio[s,t,length].

For example you can plot ratio for t at the origin and s integrated along the $x$ axis:

Plot[ratio[{x, 0}, {0, 0}, length], {x, -1, 1}]


Regarding your comment You want a function to return 1 if the shortest of all three-hop paths is shorter than $l$. Given that $n$ is finite, you could simply use If[ratio[s,t,l]==0, 1, 0]. The following function should also work (I did not test it):

shortQ[s_, t_, l_] := Block[{},
paths = {s}~Join~#~Join~{t} & /@ Subsets[pts, {2}];
lengths = Total[Norm /@ Differences[#]] & /@ paths;
If[Min[lengths] <= l, 1, 0]]

• So it looks at all paths of three hops from s to t, and gives the proportion of them less than l in Euclidean length.
– apkg
Jan 31 '18 at 21:03
• Correct. Isn't that what you are looking for? Jan 31 '18 at 21:05
• Yes indeed. What I would like to do is numerically integrate this over general $d$-dimensional point configurations, so over an $nd$-dimensional integral. This should give the CDF of the path count less than l in length.
– apkg
Jan 31 '18 at 21:08
• This works nicely. Thank you for such a quick response.
– apkg
Jan 31 '18 at 21:09
• Can this potentially be modified to give a true/false output (0/1) whenever the shortest of all the three-hop paths (the "geodesic" path) is less than L?
– apkg
Jan 31 '18 at 21:32