This function is a polynomial in two dimensions, with terms up to degree 5. It is nonlinear, and it is smooth despite being complex, which is common for computer experiment functions (Lim et al., 2002 ...

We solve polynomials algebraically in order to determine the roots - where a curve cuts the \(x\)-axis. A root of a polynomial function, \(f(x)\), is a value for \(x\) for which \(f(x) = 0\).