If X has m elements and Y has n elements, the number of onto functions are,
\(n^{m}-{n choose 1}(n-1)^{m} +{ n choose 2}(n-2^{m})……..(-1)^{n-1}{n choose n-1}1^{m}\)The formula works only
- If m ≥ n.
- If m < n, the number of onto functions is 0 as it is not possible to use all elements of Y.
