/usr/lib/gerris/libstokes3D.so is in gerris-mpi 20110329-dfsg.2-1build2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/gerris/libstokes3D-1.3.so.2.0.0
/usr/lib/gerris/libstokes3D-1.3.so.2.0.0