Some computational experiments in number theory Wieb Bosma Mathematisch Instituut Radboud University Nijmegen Nijmegen, the Netherlands
[email protected] Summary. The Magma code and some computational results of experiments in number theory are given. The experiments concern covering systems with applica- tions to explicit primality tests, the inverse of Euler’s totient function, and class number relations in Galois extensions of Q. Some evidence for various conjectures and open problems is given. 1 Introduction In the course of 10 years of working with and for Magma [6], I have conducted a large number of computational experiments in number theory. Many of them were meant, at least initially, as tests for new algorithms or implementations. In this paper I have collected results from, and code for, a few of those exper- iments. Three main themes can be recognized in the material below: covering sys- tems, the Euler φ function, and class number relations. In section 3 it is shown how covering systems can be used, with cubic reciprocity, to produce a simple criterion for the primality of n = h 3k + 1 in terms of a cubic recurrence modulo n; the starting value depends only· on the residue class of k modulo some covering modulus M. These primality tests generalize the Lucas–Lehmer type tests for numbers of the form h 2k + 1. They lead to a question about values of h for which h 3k + 1 is composite· for every k, and a generalization of a problem of Sierpi´nski.· We found a 12- digit number h with this property — a candidate analogue for the number 78577, which is most likely the smallest h with the property that h 2k +1 is composite for every k.