Permutation problem

How many invertible functions exist with the signature Za Zb if a = b? Express your answer numerically using plain text without spaces. Use / and ^ and ! to represent division, exponentiation and factorial. Use parenthesis around a term only if it is necessary. Examples: 1/(x-1), (x-3)/10, x!, (x-1)!, x^2, 2^(x-1), etc