Relative simplicial approximation
E. Christopher Zeeman · Mathematical Proceedings of the Cambridge Philosophical Society · 1964
The absolute simplicial approximation theorem, which dates back to Alexander (l), states that there is a simplicial approximation g to any given continuous map f between two finite simplicial complexes (see for instance (2), p. 37 or (3), p. 86). The relative theorem given here permits us to leave f unchanged on any subcomplex, on which f happens to be already simplicial.