Like Martha Graham in dance, Emily Post in manners, and Claire McCardell in fashion, the art dealer Edith Gregor Halpert, in her Downtown Gallery on West 13th Street, defied Eurocentrism to champion American expression, energy, and artistry. This exhibition includes approximately 100 works by Halpert’s now-famous artists, and, for the first time since its sale in 1973, her personal collection of American art and folk art is reassembled. —L.J.