Regularity of some invariant distributions on nice symmetric pairs

J.~Sekiguchi determined the semisimple symmetric pairs (g,h), called nice symmetric pairs, on which there is no non-zero invariant eigendistribution with singular support. On such pairs, we study regularity of invariant distributions annihilated by a polynomial of the Casimir operator. We deduce that invariant eigendistributions on (gl(4,R),gl(2,R)*gl(2,R)) are locally integrable functions.