Quadratic equations are polynomials, meaning strings of math terms. An expression like “x + 4” is a polynomial. They can have one or many variables in any combination, and the magnitude of them is ...

Charles K. Chui, Tian-Xiao He, Bivariate $C^1$ Quadratic Finite Elements and Vertex Splines, Mathematics of Computation, Vol. 54, No. 189 (Jan., 1990), pp. 169-187 ...