Rastrigin

f(x1,x2)=20+x12+x2210[cos(2πx1)+cos(2πx2)]f(x_1, x_2) = 20 + x_1^2 + x_2^2 - 10[\cos(2\pi x_1) + \cos(2\pi x_2)]

Contour plot

Superficie 3D