[math-fun] Tropical summation formulae

I got this from Dan Piponi (@sigfpe on twitter): sum (a+b)^-2 (c+d)^-2 (a+b+c+d)^-2 = 1/24 where the sum is over all integers (a,b,c,d) with 1<=a<=b, 1<=c<=d Paper: Tropical formulae for summation over a part of SL(2, Z) Nikita Kalinin, Mikhail Shkolnikov https://arxiv.org/abs/1711.02089 -- Tom Duff. Branches broken off trees.