# Partitions and definitions

• Jan 20th 2008, 10:12 AM
Partitions and definitions
Am i doing this right? And in the case of (c) how do you prove it?

Let B be a set.
a) Define what it means to say that $B_1 , B_2$ give a partition of B, where $B_1 , B_2$ are subsets of B.

b) Let f:A -> B be a function. Suppose that C is a subset of B. Write down the definition of $f^{-1}(C)$.

c) Suppose that $B_1 , B_2$ is a partition of B. Prove, using your definitions, that $f^{-1}(B_1) , f^{-1}(B_2)$ is a partition of A.

What i think the answers are...

a) i) $B_1 , B_2$ are non-empty
ii) since 1 is not equal to 2, ${B_1}\cap{B_2} = \phi$
iii) $B = \bigcup B_i$ The U thing goes from i=1 to n but im not sure how to do that (does n=2?)

b) $f^{-1}{C} = {(a \in | f(a) \in C)}$

c) ?

Am i right so far and if so how is c done?
• Jan 20th 2008, 10:34 AM
Plato
You are correct in parts a & b.
However, part c is not necessarily true.
If the function is surjective (onto), then it is true.

If $f$ is not surjective define $B_1 = f(A)\,\& \,B_2 = B\backslash f(A)$. That is a partition of $B$.
It is counter-example to part c.