ar X iv : 0 81 1 . 31 59 v 1 [ m at h . FA ] 1 9 N ov 2 00 8 Minimizers of the fusion frame potential ∗

In this paper we study the fusion frame potential, that is a generalization of the BenedettoFickus (vectorial) frame potential to the finite-dimensional fusion frame setting. Local and global minimizers of this potential are studied, when we restrict it to a suitable set of fusion frames. These minimizers are related to tight fusion frames as in the classical vector frame case. Still, tight fusion frames are not as frequent as tight frames; indeed we show that there are choices of parameters involved in fusion frames for which no tight fusion frame can exist. Thus, we exhibit necessary and sufficient conditions for the existence of tight fusion frames with prescribed parameters, involving the so-called Horn-Klyachko’s compatibility inequalities. The second part of the work is devoted to the study of the minimization of the fusion frame potential on a fixed sequence of subspaces, varying the sequence of weights. We related this problem to the index of the Hadamard product by positive matrices and use it to give different characterizations of these minima.