# Birkhoff–Kellogg invariant-direction theorem
**Wikidata**: [Q4916485](https://www.wikidata.org/wiki/Q4916485)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Birkhoff–Kellogg_invariant-direction_theorem)  
**Source**: https://4ort.xyz/entity/birkhoff-kellogg-invariant-direction-theorem