Revisiting ancient noncollapsed flows in ${\mathbb{R}}^3$

In this short paper, we give a new proof of the classification theorem for noncompact ancient noncollapsed flows in ${\mathbb{R}}^3$ originally due to Brendle-Choi (Inventiones 2019). Our new proof directly establishes selfsimilarity by combining the fine neck theorem from our joint work with Hershkovits and the rigidity case of Hamilton’s Harnack inequality.