# Wieferich number

> odd natural number n satisfying the congruence 2^φ(n) ≡ 1 (mod n²)

**Wikidata**: [Q125339449](https://www.wikidata.org/wiki/Q125339449)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Wieferich_number)  
**Source**: https://4ort.xyz/entity/wieferich-number