Tetration (${}^{h}a$)

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.