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Homework Help: Sorry, but I'm in dire need(Proofs)

  1. May 16, 2006 #1
    Hello, I need some help. Could someone kick me(hard please) in the right direction here? Here are the statements I need to prove:
    1) If g of f is injective, then f is injective
    2) If g of f is subjective, then g is subjective

    where g and f are functions where f:A->B and g:B -> C where A,B and C are sets

    Any kicks in the right direction would be GREATLY appreciated. Thank you.
  2. jcsd
  3. May 16, 2006 #2
    Both can be done easily by contradiction.
  4. May 16, 2006 #3


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    Homework Helper

    Start with the definitions of injections and surjections (note the spelling of the latter), and draw functional mappings (domain/codomain diagrams). The proof is fairly easy from inspecting the mappings.

    Wikipedia has fairly good pages on these subjects, complete with the mappings you need for the proof : http://en.wikipedia.org/wiki/Injective_function

    Last edited: May 16, 2006
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