Discretisation and Interpolation - The Boundary Element Method with Programming - page 38

If the element shape functions for the quadratic element are derived from Lagrange
polynomials, then there is an additional node at the centre of the element (Figure 3.10).
The shape functions are given by

L A A A B B B

(3.19) n i , j i 1 i 2 i 3 j 1 j 2 j 3

1

1

1

1

1

Figure 3.11 Serendipity and Lagrange shape functions