FactorInteger
Returns the prime factorization as {prime, exponent} pairs.
$ wo 'FactorInteger[60]'
{{2, 2}, {3, 1}, {5, 1}}
$ wo 'FactorInteger[100]'
{{2, 2}, {5, 2}}
$ wo 'FactorInteger[17]'
{{17, 1}}
$ wo 'FactorInteger[2^128 - 1]'
{{3, 1}, {5, 1}, {17, 1}, {257, 1}, {641, 1}, {65537, 1}, {274177, 1}, {6700417, 1}, {67280421310721, 1}}