Find all such mappings where $A=\{1,2,3,4\}$, $B=\{1,2,3,4,5,6\}$ and $f:A \to B$ is an injective mapping such that $f(i) \neq i ~\forall i=1,2,3,4$.
How many injective functions $f:[1,...,m]\to{[1,...,n]}$ has no fixed point? $(m\le n)$