Mitchell Innes & Nash 534 West 26th Street , Chelsea , and 1018 Madison Avenue , near 78th Street , Manhattan Through Nov. 12 Gagosian Gallery 555 West 24th Street , Chelsea Through tomorrow The greatness of Roy Lichtenstein 's Pop Art , with its irresistibly suave mixture of high and low images , techniques and styles , is confirmed but not substantially revised by these three displays .