# Leave boundary dofs out of matrix construction

I'm implementing a Navier--Stokes solver for axisymmetric geometries, and in the weak forumulation, the term

u[0] * v[0] / r^2

appears (cf. <https:/

In Dolfin, the resulting matrix would look like

[I 0 ] [u_r] = [0]

[b A] [u ] [f]

where b corresponds to the terms where u[0] is nonzero at the boundary, and thus b would contain NaNs. They don't play a particular role since u_r is 0 there, but I don't if every backend picks that up. Also, applying some sort of LU solver, in fact any method that does something with the matrix entries (e.g., AMG), is asking for all sorts of trouble.

Question:

Is is possible to restrict the trial space as well to directly get an equation system of the form

[A][u] = [f]

?

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- Garth Wells

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