It's like the Fibonacci Sequence, but a lot more computationally expensive and a lot less useful.
exps(x, y, z) =
if x <= 0: y + z
if x > 0: exps(x-1,y+1,z) + exps(x-2,y+2,z*2) + exps(x-3,y+3,z*3)
x, y, and z are all greater than or equal to zero.
This will be tested on inputs as large as:
x = 150
y = 400
z = 800
Use a hashtable to make sure your solution completes before the universe ends.
Hint: Va Clguba, n qvpg xrl pna or nal vzzhgnoyr glcr... vapyhqvat n ghcyr.
(That's encrypted with ROT13--Google rot13 decoder to decode it if you
want the hint.)