## The self-power map and collecting all residue classes

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- by Catalina V. Anghel;
- Math. Comp.
**85**(2016), 379-399 - DOI: https://doi.org/10.1090/mcom/2978
- Published electronically: May 12, 2015
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## Abstract:

The self-power map is the function from the set of natural numbers to itself which sends the number $n$ to $n^n$. Motivated by applications to cryptography, we consider the image of this map modulo a prime $p$. We study the question of how large $x$ must be so that $n^n \equiv a \bmod p$ has a solution $1 \le n \le x$, for every residue class $a$ modulo $p$. While $n^n \bmod p$ is not uniformly distributed, it does appear to behave in certain ways as a random function. We give a heuristic argument to show that the expected $x$ is approximately $p^2\log \phi (p-1)/\phi (p-1)$, using the coupon collector problem as a model. We prove the bound $x <p^{2-\alpha }$ for sufficiently large $p$ and a fixed constant $\alpha > 0$ independent of $p$, using a counting argument and exponential sum bounds.## References

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## Bibliographic Information

**Catalina V. Anghel**- Affiliation: Department of Mathematics, University of Toronto, 40 St. George St., Toronto, ON M5S 2E4, Canada
- Email: catalina.anghel@alum.utoronto.ca
- Received by editor(s): April 21, 2013
- Received by editor(s) in revised form: April 22, 2014
- Published electronically: May 12, 2015
- © Copyright 2015 American Mathematical Society
- Journal: Math. Comp.
**85**(2016), 379-399 - MSC (2010): Primary 11A07, 11K45, 11T23; Secondary 94A60
- DOI: https://doi.org/10.1090/mcom/2978
- MathSciNet review: 3404454