Simple homotopy of flag simplicial complexes and contractible contractions of graphs
Anton Dochtermann, Takahiro Matsushita · arXiv (Cornell University) · 2023
A simplicial complex $X$ is flag if it is isomorphic to the clique complex of some graph. Boulet, Fieux, and Jouve have shown that simple homotopy equivalence classes of such complexes are determined by the notion of $s$-homotopy defined on its $1$-skeleton. In his work on molecular spaces, Ivaschenko introduced the notion of $\mathfrak{I}$-contractible transformations on finite graphs, and we say that two graphs are $\mathfrak{I}$-homotopy equivalent if there is a sequence of $\mathfrak{I}$-contractible transformations that takes one to the other. We show that the $s$-homotopy type and $\mathfrak{I}$-homotopy type of a finite graph coincide, answering a question posed by Boulet, Fieux, and Jouve. As a result, two graphs $G$ and $H$ are $\mathfrak{I}$-homotopy equivalent if and only if their clique complexes $C(G)$ and $C(H)$ are simple homotopy equivalent. We also show that a finite graph $G$ is $\mathfrak{I}$-contractible if and only if $C(G)$ is contractible, which answers a question posed by the first author, Espinoza, Frías-Armenta, and Hernández.