Projected and near-projected embeddings

TL;DR

This paper distinguishes between $k$-realizable and $k$-prem smooth maps, refuting a long-standing conjecture by providing counterexamples and establishing conditions under which the two notions coincide.

Contribution

It constructs explicit counterexamples showing that $k$-realizable does not imply $k$-prem, and proves the equivalence in various broad cases.

Findings

Counterexamples for $k$-realizable not implying $k$-prem when $n=4k+3 ext{ and }n ext{ large}$

Conditions under which $k$-realizable maps are $k$-prem, including bounds on $q$ and $k$

Refutation of the conjecture that all $k$-realizable maps are $k$-prem

Abstract

A stable smooth map $f:N\to M$ is called "$k$-realizable" if its composition with the inclusion $M\subset M\times\Bbb R^k$ is $C^0$-approximable by smooth embeddings; and a "$k$-prem" if the same composition is $C^\infty$-approximable by smooth embeddings, or equivalently if $f$ lifts vertically to a smooth embedding $N\to M\times\Bbb R^k$. It is obvious that if $f$ is a $k$-prem, then it is $k$-realizable. We refute the long-standing conjecture that the converse is always true. Namely, for each $n=4k+3\ge 15$ there exists a stable smooth immersion $S^n\to\Bbb R^{2n-7}$ that is $3$-realizable but is not a $3$-prem. We also prove the converse in a wide range of cases. A $k$-realizable stable smooth fold map $N^n\to\Bbb R^{2n-q}$ is a $k$-prem if $q\le n$ and $q\le 2k-3$; or if $q<n/2$ and $k=1$; or if $q\in\{2k-1,\,2k-2\}$ and $k\in\{2,4,8\}$ and $n$ is sufficiently large.