ffc team mailing list archive

[Kristian Oelgaard <k.b.oelgaard@xxxxxxxxxx>]

Quoting Shawn Walker <walker@xxxxxxxxxxxxxxx>:

> Hello.  I need to know how FFC orders P2 Lagrange nodes.  I need to know
> this in order to compare a FEM matrix that I created in another program.
>
> For example, say you have one triangle with the following nodes:
>
> [0 1 2 3 4 5]
>
> Where is the #3 node in relation to the #0, #1, and #2 nodes?  Is #3
> opposite the #0 node?

Yes.

I usually look at the tabulate_dofs() function, for the P2 case:

dofs[0] = c.entity_indices[0][0];
dofs[1] = c.entity_indices[0][1];
dofs[2] = c.entity_indices[0][2];
unsigned int offset = m.num_entities[0];
dofs[3] = offset + c.entity_indices[1][0];
dofs[4] = offset + c.entity_indices[1][1];
dofs[5] = offset + c.entity_indices[1][2];

So you see that the first 3 dofs are the vertices, the next 3 are the midpoints
of the edges. Then you need to know the UFC ordering of edges to figure out
that e0 is the edge which does not contain vertex 0 (opposite to v0) etc.

Kristian

> - Shawn
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