“…For instance, cloaking devices, [14][15][16] flat lenses based on negative refraction, 17,18 gradient index flat and circular lenses [19][20][21] and omnidirectional absorbers 22,23 for flexural waves have been recently proposed and experimentally verified. Also, the dispersion relation of plates with periodic arrangements of rigid pins, 24,25 holes, 26 attached pillars [27][28][29] or point-like spring-mass resonators [30][31][32] attached to them has been investigated by several groups, as well as the resonant properties of complex inclusions. 33 In the aforementioned works, the properties of the complex structure are mainly determined by band structure calculation, since an effective medium theory for the description of these new metamaterials, specially relating the different resonant parameters with the corresponding symmetry of the field, has not been given so far, as has been done for acoustic or bulk elastic waves.…”