 Tetration

Repeated exponentiation with right association: ${}^{1}a=a$ and ${}^{h+1}a=a^{{}^{h}a}$. For example, ${}^{3}a=a^{a^a}$, rather than $(a^a{)}^a$. Computing its remainder modulo an integer often uses a chain of smaller exponent moduli; applying Euler's theorem requires checking its coprimality hypothesis at every level.

Tetration is a mathematical operation that involves exponentiation in a repeated fashion. Specifically, it is the iteration of exponentiation, just as multiplication is the iteration of addition and exponentiation is the iteration of multiplication.