Ms. McKellar, now a semiregular on "The West Wing" playing a White House speechwriter, Elsie Snuffin, is probably the only person on prime-time television who moonlights as a cyberspace math tutor.
Her mathematics knowledge extends well beyond calculus. As a math major at the University of California, Los Angeles, she also took more esoteric classes, the ones with names like "complex analysis" and "real analysis," and she pondered making a career move to professional mathematician.
"I love that stuff," Ms. McKellar said last month during a visit to Manhattan after a play-reading in the Hamptons. Her conversation was peppered with terminology like "epsilons" and "limsups" (pronounced "lim soups").
"I love continuous functions and proving if functions are continuous or not," she said.
She may also be the only actress, now or ever, to prove a new mathematical theorem, one that bears her name. Certainly, she is the only theorem prover who appears wearing black lingerie in the July issue of Stuff magazine. Even in that interview, she mentioned math.