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Tribonacci numbers and primes of the form p = x2 + 11y2

  • Tim Evink EMAIL logo and Paul Alexander Helminck
Published/Copyright: May 21, 2019
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Abstract

In this paper we show that for any prime number p not equal to 11 or 19, the Tribonacci number Tp−1 is divisible by p if and only if p is of the form x2 + 11y2. We first use class field theory on the Galois closure of the number field corresponding to the polynomial x3x2x − 1 to give the splitting behavior of primes in this number field. After that, we apply these results to the explicit exponential formula for Tp−1. We also give a connection between the Tribonacci numbers and the Fourier coefficients of the unique newform of weight 2 and level 11.

  1. (Communicated by Filippo Nuccio)

Acknowledgement

The authors would like to thank Associate Professor Burkard Polster (from “Mathologer”) for bringing these numbers under their attention through his video on YouTube on Tribonacci numbers. The authors would also like to thank Prof. Jaap Top for pointing out the connection with modular forms and the unique newform of weight 2 and level 11 and the referees for their comments and remarks, in particular for suggesting Lemmas 3.1 and 3.2.

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Received: 2018-03-28
Accepted: 2018-07-22
Published Online: 2019-05-21
Published in Print: 2019-06-26

© 2019 Mathematical Institute Slovak Academy of Sciences

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