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For the numerical integration of reaction terms in my PDE on a 2D triangular mesh, I would like to use the scheme $$ \int_T \Phi(x) \approx \frac{|T|}{3} (\Phi(x_0) + \Phi(x_1) + \Phi(x_2)) $$ where the $x_i$ are the triangle nodes.

How to specify this when assembleing the system?

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Actually you don't specify quadrature rule to assembler but to form compiler. You can tweak ffc/quadrature_schemes.py for your needs or you can adjust generated code. In both cases you just supply your quadrature points and weights.

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  • $\begingroup$ Luckily, assemble() has an form_compiler_parameters argument. $\endgroup$ – Nico Schlömer May 17 '13 at 13:25
  • $\begingroup$ @Nico: Yes, but FFC seems to accept only default and canonical which is a scheme supplied by FIAT. $\endgroup$ – Jan Blechta May 17 '13 at 13:28
  • $\begingroup$ Indeed, quadrature_schemes.py doesn't seem to permit custom schemes. $\endgroup$ – Nico Schlömer May 17 '13 at 13:55
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You can adjust the quadrature degree by setting

parameters["form_compiler_parameters"]["quadrature_degree"] = q

This will change the quadrature degree such that polynomials of degree q are integrated exactly.

FEniCS will try to autodetect the necessary quadrature degree so this should normally not be needed. However, in some cases you might want to underintegrate by setting a lower value of q.

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  • $\begingroup$ I skimmed through FFC/ffc/quadrature_schemes.py to see what schemes are used for the particular degrees, but the "lumping" scheme that I'd like to use is not implemented. $\endgroup$ – Nico Schlömer May 17 '13 at 13:54

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