You can get all DoF indices on a particular boundary via
DoFTools::extract_boundary_dofs(). You can correlate this
DoFTools::extract_dofs() with a component mask that corresponds to your
x-displacement. With the information produced by these two you should be able
to pick out which DoFs you want to work on.
Best
W.
--
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Wolfgang Bangerth email:
bang...@colostate.edu
www:
http://www.math.colostate.edu/~bangerth/