Q2-P1 is one of the widely-used mixed elements for Stokes flow, and to a little extent for solid mechanics.

What is the function space of the basis functions for the pressure field for this Q2-P1 element? Some papers assume it as bi-linear (1+ax+by+cxy) and others consider it to be linear ( 1+ax+by only, no product term). This makes a significant difference in computer implementation.

I want to know which one is correct.

Thanks in advance!

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