isoparametric-quadrilateral
Two dimensional shape functions for "square", with one function per corner, n0, n1, n2, n3. Where the portion of the four functions is such that: at (-1, -1) n0 is 1, and at all other corners is 0, at ( 1, -1) n1 is 1, and at all other corners is 0, at (-1, 1) n2 is 1, and at all other corners is 0, and at ( 1, 1) n3 is 1, and at all other corners is 0. The functions are of the form c0 + c1*s + c2*t + c3*s*t where c0, c1, c2, c4 are the constants for that particular function