THE EXPANSIVE ABSTRACT PAINTINGS in Buy Shaver's recent show display varied theoretical strategies: Some are absorbed in purely perceptual investigations, while others give a quick nod to formalism before heading off into the territory of ambiguity and metaphor. Among the conventionally strict examples is B.T.M.A.O. (White Square), 1998, a small canvas consisting of enameled rectangles of color in a lateral, bannerlike rhythm interrupted by a single insistent white square. A more animated perceptual jolt is caused by Red Flag, 2000, in which four distinct sections of gridded black and red meet