use feigen gives material, stiffness, modes, name_modes and
plot_modes. The element comes from fortran_cubatures (cubature) and
fortran_shapefuncs (shapefunc). The feigen program wraps them in a
command line. Start with the user manual, which is also
available as plain text in llms.txt.
q = cubature(CUB_QUA, 1) ! one point: reduced integration
s = shapefunc(q, 1) ! 4-node quadrilateral
K = stiffness(s, q, s%coords, material(2, 1._dp, 0.3_dp))
call modes(K, lambda, phi)
print *, count(lambda == 0) ! 5: 3 rigid-body + 2 hourglass