How to prove that A $\subseteq$B $\implies$ |A|$\le$|B|? I know that for |A|$\le$|B| there has to be a function f:A $\mapsto$B which is an injective function. But i get stuck because the sets A and B were not specified, so should i just let A and B be anything i decide ?
2026-04-13 06:36:15.1776062175
How to prove that A $\subseteq$B $\implies$ |A|$\le$|B|?
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Consider the inclusion function:
$$f:A\to B,\quad f(a)=a\quad\forall a\in A$$