# Proving a certain function is injective

• Dec 21st 2009, 05:20 PM
Pinkk
Proving a certain function is injective
Let $\displaystyle A$ and $\displaystyle B$ be nonempty sets such that $\displaystyle |A| < |B|$. Show there exists an injective function from $\displaystyle \mathcal{P}(A)$ to $\displaystyle \mathcal{P}(B)$.

This was a question on my final and luckily I was able to omit it because I wasn't sure how to tackle this problem, but I'm curious to what the answer is. Thanks!
• Dec 21st 2009, 05:57 PM
NonCommAlg
Quote:

Originally Posted by Pinkk
Let $\displaystyle A$ and $\displaystyle B$ be nonempty sets such that $\displaystyle |A| < |B|$. Show there exists an injective function from $\displaystyle \mathcal{P}(A)$ to $\displaystyle \mathcal{P}(B)$.

This was a question on my final and luckily I was able to omit it because I wasn't sure how to tackle this problem, but I'm curious to what the answer is. Thanks!

so there exists an injection $\displaystyle f: A \to B.$ noe define $\displaystyle g :\mathcal{P}(A) \to \mathcal{P}(B)$ by $\displaystyle g(X)=f(X)=\{f(x): \ x \in X \},$ for all $\displaystyle X \in \mathcal{P}(A).$