Thread: injective functions

i got confused by this question seems relatively simple but dnt get it.
how many non-injective functions are there from {1,2,3,4} to {1,2,3,4,5}? How many non-injective functions are there from {1,2,3,4,5} to {1,2,3,4}?

thanks.

Using a usual notation for sets of functions , then if there are functions from A to B. Of those are injections. So how many non-injections are there?

There are no injections from B to A. WHY?

because B has more elements than A
when you say (5)(4)(3)(2)..what do you mean.thanks

Originally Posted by skystar

when you say (5)(4)(3)(2)..what do you mean.thanks

Multiply: (5)(4)(3)(2)=120!