如何在不完全禁止公式块中的分页符的情况下阻止分页符？

Question

喂食

\documentclass{article}
\predisplaypenalty=0%%% that's what our class svmono.cls from a nonminimal example (not given here) does.
\usepackage[helvratio=.9]{newtxtext}
\usepackage[slantedGreek,subscriptcorrection]{newtxmath}
\usepackage{mathtools}
\mathtoolsset{mathic=true}
\begin{document}
Begin.\\[101.9ex]%%% with 101.8ex, the f-block is on p.1.  With 101.9ex, the f-block is on p.2
Consider the following functions:
\begin{align*}
  f\colon \mathbb{N}&\to \mathbb{N}\\% allow for a page break after