1The easiest way to define ϕk is by making use of the barycentric coordinates λi(x,y), i = 1, 2, 3 of a point q = (x,y) T,
defined by
where qi, i = 1, 2, 3 are the 3 vertices of T. Then it is easy to see that the restriction of ϕk on T is precisely λk.