Eliminating higher-multiplicity intersections in the metastable dimension range

Abstract

The procedure to remove double intersections called the Whitney trick is one of the main tools in the topology of manifolds. The analogues of Whitney trick for $r$-tuple intersections were `in the air' since 1960s. However, only recently they were stated, proved and applied to obtain interesting results. Here we prove and apply the $r$-fold Whitney trick when general position $r$-tuple intersection has positive dimension. A continuous map $f\colon M \to B^d$ from a manifold with boundary to the $d$-dimensional ball is called proper, if $f^{-1}(\partial B^d)=\partial M$. Theorem. Let $D=D_1\sqcup\ldots\sqcup D_r$ be disjoint union of $k$-dimensional disks, and $f:D\to B^d$ a proper map such that $f\partial D_1\cap\ldots\cap f\partial D_r=\emptyset$, and the map $$f^r:\partial(D_1\times\ldots\times D_r)\to (B^d)^r-\{(x,x,\ldots,x)\in(B^d)^r\ :\ x\in B^d\}$$ extends continuously to $D_1\times\ldots\times D_r$. If $rd\ge (r+1)k+3$, then there is a proper map $\bar f:D\to B^d$ such that $\bar f=f$ on $\partial D$ and $\bar fD_1\cap\ldots\cap \bar fD_r=\emptyset$.