Friday, 11 April 2014

elementary set theory - Injective and surjective functions in ℕ

Let f:NN be an injective function.



Let g:NN be a surjective function



prove that f(n)g(n) for all nN.




this exercise has been puzzling me a long time.
the most reasonable proof would by finding a contradiction, and by proving that surjective functions from NN Would have to either be an identity function or a function that assigns different values to even and odd numbers ( I could be wrong)
I tried playing a bit with the properties of injective and surgective functions since an injective function in N would have to be strictly increasing.
p.s : I'm still a highschooler so I'm fairly ignorant.

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