\(f(n) = ((n/2)+1)^2\)
You could go on to prove that this is a bijection. However, this late in the term, we'll take this as obvious.
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.