On 7/23/20 12:07 PM, Xuefeng Li wrote:

Well, the above function calculates the gradients of a finite element at thequadrature points of a cell, not at the nodal points of a cell.Such a need arises in the following situation. for ( x in vector_of_nodal_points ) v(x) = g(x, u(x), grad u(x))

`It's worth pointing out, however, that for the common FE_Q elements, the`

`function values u(x) are continuous and so it doesn't matter how exactly you`

`compute u(x) at node points. On the other hand, grad u(x) is in general`

`discontinuous and so trying to evaluate it at node points is not actually`

`possible: You will either get the values from one adjacent cell or the value`

`from another.`

`In other words, if you want to compute a function that depends on 'grad u',`

`you need to think about what exactly you mean by that. In the formulation`

`above, v(x) will in general be a discontinuous function, and you need to think`

`about whether using FE_Q (a continuous finite element space) is really what`

`you want to do.`

Best W. -- ------------------------------------------------------------------------ Wolfgang Bangerth email: bange...@colostate.edu www: http://www.math.colostate.edu/~bangerth/ -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en

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