On the last digits of tetrations of base $2^k$ and $5^k$

Abstract

In this paper will be proved the existence of a formula to reduce a tetration of base $2^k$ and $5^k$ $\mathrm{mod}10^n$. Indeed, last digits of a tetration are the same starting from a certain hyper-exponent; In order to compute the last digits of those expressions we reduce them $\mathrm{mod}10^n$. Lots of different formulas will be derived, for different cases of $k$ (where $k$ is the exponent of the base of the tetration). This kind of operation is fascinating, because the tetration grows very fast. But using these formulas we can actually have informations about the last digits of those expressions. It's possible to use these results on a software in order to reduce tetrations $\mathrm{mod}10^n$ faster.

arXiv commentThe formula provided in section "5^k" doesn't hold for all cases. There are some errors also in section "2^((5^t)k)". In the paper "Observations regarding the repetition of the last digits of a tetration of generic base" we analyze those formulas and we provide a more precise conjecture