## Complexity of untying $$2$$-knots

Boris Lishak (Sydney)

Abstract

Embeddings of $$S^2$$ into $$S^4$$ can be topologically nontrivial. But even if the embedding is trivial, geometrically it can "look" knotted. We will give examples of topologically trivial $$2$$-knots such that their complexity will grow super-exponentially. Complexity will be either defined in terms of Riemannian geometry in the smooth case or combinatorially if the embedding is piece-wise linear. Proofs will use some group theory.