A function f(x)f(x)f(x) is even if:
f(−x)=f(x)f(-x) = f(x)f(−x)=f(x)
f(−x)=−f(x)f(-x) = -f(x)f(−x)=−f(x)
f(x)=1/f(x)f(x) = 1/f(x)f(x)=1/f(x)
f(x)f(x)f(x) is a polynomial with even degree