MAME COHALAN retreats to the zoo these days, filling the small mammal house with painted trees and trunks, deciding where the rocks go. She thinks her best interior work is the python cage where airbrushed snakeskins line the walls.

Her outdoor experiments in higher mathematics draw more notice.

A mystical vista opens on Seventh Street and Indiana Avenue, opposite the oppressive National Archives building. Is it Italian Renaissance? Islamic, Medieval or Athenian? It's more Disneyworld, according to Cohalan, who designed and painted the mural with an $8,000 grant, employing 25 high school students for minimum wage. On close inspection, it's a matter of perspective. Not great art so much as a giant joke.

Opposite the National Gallery, at Fourth and Pennsylvania, in another Cohalan trompe l'oeil, louvers of color, horizons of sea and sky, disassemble and shift with the viewer's angle. You can tell there's serious math going on here: left to right, blue and brown checked tiles, brown and yellow border, five walls of red, green, blue. Right to left, walls of bright yellows, pinks, smooth ceiling and base. Up close, each strip is a "V," with two painted sides jutting out from the frame. Reynolds Metals Co. donated 4,000 pounds of aluminum for the 16-by-60-foot work. The artist makes no apology for its derivation: She saw "all the Midwesterners transfixed" by Yaacov Agam's "Transparent Rhythms II" (1967-68) at the Hirshhorn and began taking notes.

Last summer, an enigmatic, Mediterranean complex of roofs, arches and silhouette figures appeared on the side of the Goldberg Marchesano public relations firm townhouse on Sunderland Place at 20th Street. Cohalan again, daring to include people in the drawing for the first time. She'd like to improvise new openings for more characters in the Marrakesh-like scene and fix a couple of mistakes that only she would notice.

Carol Marchesano, president of the firm, had final approval over the design and nearly chose an antique advertisement, the Old Mail Pouch Tobacco ad with the wall painted like the side of a barn. "But we decided to be more civic-minded," she says. During the painting process, "I had visions of accidents and lawsuits, and would come in every day to see putrid colors clashing. Mame gave the kids freedom to experiment and see how bad color combinations could be." The firm spent close to $7,000 for the design and a CETA grant covered the rest.

Cohalan calls mural painting her penance in life, and worries that the days of CETA funding are over, just when she'd mastered the paperwork. But now, with support from the Pennsylvania Avenue Development Corp., she's about to patch the colonnated arches, spires and dome on the air vents and walls. The mural's flaky state of disrepair is "a humiliation . . . they mixed vodka or something into the concrete," she says. With donations from a scaffolding company and a paint supplier, she'll begin touch-ups shortly. The mural's safe at least until the proposed Navy Arch takes shape in the park. "No one's said a word to me about whether they consider that crazy mural to be complementary or destructive to the arch," she said.

Cohalan's career has evolved with a certain logic. Her background in theater--directing and acting at Catholic University and on stage briefly at Arena--foreshadowed her painted stage sets. When she left the theater, though, it was to delve into theoretical mathematics. In effect, she began painting to solve a math problem. Hers is the art of prime number theory, with her architect husband Michael offering technical support. Unschooled as an artist, Cohalan lacked the connections she felt would grant her entry to the gallery world, so she turned to public art.

Cohalan is an impish 43, her wide eyes matching her green sweatshirt. Her temperament may be childlike, but she adopts the requisite business mentality to play the role of grant- seeking muralist. It's the side of her that hustles lawyers, insurance companies, foundations and corporations that "makes me put on a gray suit to go to the Fine Arts Commission."

Ultimately, street art makes for fleeting fame. While chastising herself for not inventing a weatherproofing technique, Cohalan accepts the time limits on her works. Besides the lifespan of paint, the art itself "develops an eventual distaste in its watchers," she admits. Ideally, she figures, a work should live for three to five years before it's painted over by another artist.