Question

How many one-to-one functions are there from 4-byte words to 4-byte words? How many onto
functions?

Answer 1

{{a, b, c, d}, {a, b, d, c}, {a, c, b, d}, {a, c, d, b}, {a, d, b,
c}, {a, d, c, b}, {b, a, c, d}, {b, a, d, c}, {b, c, a, d}, {b, c,
d, a}, {b, d, a, c}, {b, d, c, a}, {c, a, b, d}, {c, a, d, b}, {c,
b, a, d}, {c, b, d, a}, {c, d, a, b}, {c, d, b, a}, {d, a, b,
c}, {d, a, c, b}, {d, b, a, c}, {d, b, c, a}, {d, c, a, b}, {d, c,
b, a}}

Answer 2

4!=24