(a-b^2)^2

Expanded form: a^2 − 2a b^2 + b^4

Reason: (a − b^2)^2 = a^2 − 2a(b^2) + (b^2)^2 = a^2 − 2ab^2 + b^4

It's a perfect square; nonnegative for all real a, b, with minimum 0 when a = b^2.