# Closed formula for two variable recurrence

I would like to know if there exists a closed form formula to the following recurrence:

• $$f(s, 0) = 1$$
• $$f(s,b) = \displaystyle\sum_{i=1}^{min(s, b)} \left[ (s-i+1)\times f(i, b-i) \right]$$

This recurrence gives the solution to problem F of the ICPC Latin American regionals of 2019.

The problem itself can be solved in $$O(n^2)$$ by using dynamic programming and keeping a matrix of precomputed sums. I'm just curious about if there is some technique that allows obtaining a closed formula to recurrences like this, so I can solve it in better runtime complexity.

I do not have a strong mathematical background, but I am afraid that recursive functions cannot always been written in closed form.

ex1.

f(n) = sum([1,2,..,n]) can be written as
f(1) = 1
f(n) = n + f(n-1)

Of course f(n) = n*(n+1)/2

ex2.

f(n) = prod([1,2,3,4,..n]) = n!
f(1) = 1
f(n) = n * f(n-1)

No known closed formula :( (I don't even know how to prove something like this)

In your case, I find it extremely difficult because your recursion makes use of an if statement under the min() function.