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

∑         ∑
   λi = 1,   λiqi = q
 i         i

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.