# Math Help - finite cardinality proof

1. ## finite cardinality proof

Prove that {1,...,10}X{1,...,15} is finite using only the definition of a finite set.
Defn.: "a set S is finite if either S is the empty set (well obviously not!) OR S is equivalent to the set {1,2,3,...,n} for some positive integer n.

2. Originally Posted by dolphinlover
Prove that {1,...,10}X{1,...,15} is finite using only the definition of a finite set.
Defn.: "a set S is finite if either S is the empty set (well obviously not!) OR S is equivalent to the set {1,2,3,...,n} for some positive integer n.
Consider $fa,b) \to \left[(a-1)+10\times (b-1) +1\right] " alt="fa,b) \to \left[(a-1)+10\times (b-1) +1\right] " />

Then the smallest value $f$ takes is $1$, and the largest is $150$, the image of distinct elements of $\{1,...,10\}X\{1,...,15\}$ are distinct. Thus we have $150$ points in the range and $150$ points in the image, and the image is: $\{1, ..., 150\}$.

Now turn this into a formal demonstration.

CB

3. Originally Posted by Mathstud28
Well if $f:[1,10]\mapsto[1,15]$ your domain would be $[1,10]$ and your codomain would be $[1,15]$. So now before I say something that is unclear again, do you know what either one-to-one or mapping means? Do you understand what it means if two sets are equivalent, denoted $A\sim{J}$
Yes I do understand 1-to-1 & onto (mapping)...Most of our proofs in the last couple weeks have involved bijection....how does this relate to whether or not it's finite (or infinite)...I have already defined my domain & codomain (see previous entry)... f: domain--->codomain. What is f?...graphically it's a solid rectangle therefore how is it 1-to-1? (the cartesian product)....I need help!