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 ...

The key features of a quadratic function are the y-intercept, the axis of symmetry, and the coordinates and nature of the turning point (or vertex). Part of Maths Algebraic skills ...