# types of functions

• Feb 4th 2007, 06:45 PM
luckyc1423
types of functions
(a) Suppose that the set A has exactly two elements and the set B has exactly three. You have to construct explicitly all functions from A to B, and to decide whether each of them is injective and surjective. There are nine such functions – let us label them (arbitrarily) as f1, f2, . . ., f9.
Let A = {1, 2}, and B = {a, b, c}. In the table below fill out all possible values of f1,
f2, . . ., f9 on 1 and 2.

Function fi(1) fi(2) Injective? Surjective?
f1 a a No No
f2 a b Yes No
f3 a c Yes No
f4 ? ? ? ?
f5 ? ? ? ?
f6 ? ? ? ?
f7 ? ? ? ?
f8 ? ? ? ?
f9 ? ? ? ?

(b) Now let A has exactly three elements and B has exactly two: A = {1, 2, 3}, B = {a, b}.
Again, construct explicitly all functions from A to B and determine whether they are injective and surjective. Put your results in a table similar to the table in part (a).
(c) Let the set A have exactly m elements and B have exactly n (where n,m 2 N). How many different functions are there from A to B?
• Feb 4th 2007, 07:13 PM
Plato
If |A|<|B| there are NO surjections from A to B and permut(|B|,|A|) injections.

The number of mapping from A to B is $\left| B \right|^{\left| A \right|}.$
• Feb 4th 2007, 07:16 PM
luckyc1423
Right, I know all the rules, I have this in my notes....I am still having problems with this problem because there are no other similiar examples in my book..
• Feb 4th 2007, 07:38 PM
ThePerfectHacker
Quote:

Originally Posted by Plato
If |A|<|B| there are NO surjections from A to B and permut(|B|,|A|) injections.

The number of mapping from A to B is $\left| B \right|^{\left| A \right|}.$

What happens if $|B|=3$ and $|A|=\aleph_0$. Then what is $3^{\aleph_0}$?

Note, It is understandable to me when $|B|=2$ then $2^{\aleph_0}=|\mathcal{P}(A)|$