# FEM: weak forms

1. Oct 30, 2009

### defunc

For a second order differential equation, is it necessary to derive a weak form if quadratic interpolation are used?

2. Oct 30, 2009

### ank_gl

For the weak form of a second order differential equation, both weighting function(w) & the primary variable(u) are required to have non zero first order derivatives(in order to define the stiffness matrix), ie linear approximation can suffice provided the problem allows. Linear elements will not provide interelement continuity though, hence continuity of secondary variables cannot be guaranteed . Quadratic elements provide SV continuity.