“…To study extremal properties of the spectrum, it is therefore necessary to impose additional constraints, either of an intrinsic or an extrinsic nature. For example, we can assume that the induced metric g preserves a conformal class of metrics [7,12,20], a symplectic or a Kähler structure [2,27], the action of a Lie group [1,6,14], etc. Regarding results with constraints of extrinsic type, a well-known example is given by Reilly's inequality [13,29]:…”