2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49918We review some old and new methods of reduction of the number of degrees of freedom from ~N^2 to ~N in the multi-matrix integrals.16 pages, a talk delivered at the MSRI Workshop ``Matrix Models and Painlevé Equations'', Berkeley (USA) 1999High Energy Physics - Theorytext