# Countability Problem 1

### Solution

\(f(n) = ((n/2)+1)^2\)

You could go on to prove that this is a bijection. However, this late in the
term, we'll take this as obvious.

### Self-check

There might be other solutions to this problem. (E.g. perhaps you
could define a different function by using multiple cases?) However,
if you have a simple algebraic formula different from the above, it's
probably wrong. Try testing it by putting some specific members of A
into the definition.