Solution
= Solution
An <injective function> satisfies $f(a)=f(a')\Rightarrow a=a'$ for all $a,a'\in A$. A <surjective function> $g:A\to B$ satisfies: for every $b\in B$, some $a\in A$ has $g(a)=b$. The following arguments apply to <function composition> with the stated domains and codomains.