Jennifer Jason Leigh has joined the cast of David Auburn's play about the mysterious nature of genius, the Pulitzer Prize-winning Proof

At the very close of Proof, David Auburn's Pulitzer Prize-winning play about fathers, daughters, death, dementia and a few other things, its stubborn 25-year-old heroine, a mathematically brilliant young woman named Catherine, is disclosing to one of her late father's former students — a fellow who's falling in love with her, but has doubted her — some of the thought processes of her hidden arithmetical explorations. She looks at him for a moment, and then, opening a notebook at random, says:

"Here. I've got Eberhart's Conjecture setting up this section, qn as the nth prime, all that stuff, b's a positive not divisible by p . . . Pretty basic number theory. It just seems wrong to be using it to get to the Gauss. I'd like to go around, but when you eliminate it you get contradictions, or everything goes to zero. Unless . . ."

So it seemed only reasonable to ask Jennifer Jason Leigh, who is now playing Catherine: How's your math? "Comme ci, comme ca," said the actress who had dropped out of high school a few weeks short of graduation to take her first big role in a movie, the 1981 Eyes of a Stranger. "I was very good at algebra. Geometry, not so good."

Big as a minute, serious as a dormouse, wide awake as a firecracker, attractive as sin, she nursed an iced tea and said: "I'm going to this NYU professor's lecture next week. I'm sure I won't understand a word. He said he's going to introduce me to some teachers of number theory." Two beats. "Actually, I think my performance got better after I talked with him . . . He told me that when you write a proof, you should show it to people and ask them to tear it to shreds. Exactly the opposite of acting," she observed. "When you're in trouble as an actor, you ask for help from your friends. That's the difference between acting and math." It is to be doubted that Jennifer Jason Leigh's performance as Catherine needed a shot in the arm from a numbers theorist to get any better than it already is, and was. She has put into that performance every bit of the vitality, vulnerability, rage and high intelligence that has stamped her work all the way from the pregnant teen-ager of Fast Times at Ridgemont High (1982) to the gang-raped Tralala of Last Exit to Brooklyn (1989) to the strung-out rebellious kid sister of Georgia (1995) — which won her the Best Actress award at Montreal's World Film Festival— to the Sally Bowles of Broadway's Cabaret, in the wake of Natasha Richardson, to, now, her Catherine in Proof.

"Whenever you take over a part," the new Catherine says, "it's always a challenge to make it your own. I was back in New York from the Taormina Film Festival when I got a call: 'Would you be interested in going into Proof ?' Mary-Louise Parker really did it in such a spirited and unique way, it gave me pause. At first I said no, I had all these prior obligations. But saying no made me sad. It would be such a joy to do every night; it haunted me.

"Then I met [director] Dan Sullivan that weekend. He said: 'I wouldn't ask you to do it if we weren't going to reexplore and reinvent the play.' That's when I said yes. You know, people are always saying there are so few great parts for women. I say there are so few great parts, period.

"I want to do more theatre. Especially at a time like this, when it's been so helpful. You lose yourself for two hours every night, and I think the audience feels that, too. In a way, you do feel that you're sharing the experience together. Catherine in the play is going through such grief and shock and rage" — over the loss of her father, first to his mental impairment, then literally — "that everyone's feeling it right along with her."

Jennifer Jason Leigh's mother is Barbara Turner, who wrote the screenplay of Georgia; her father was Vic Morrow, the Bronx-born actor most vividly remembered as that really rotten high-school punk in Blackboard Jungle. Her parents divorced when Jennifer was two; Morrow was killed in a helicopter accident when she was 20. She nudged her mother into writing Georgia "because I was so desperate to work with her. I just came up with an idea for a movie about sisters," says the movie star who has an older sister Carrie and a younger half-sister Mina Badie. Jennifer and her mother have a new project in the works: "a movie about an off-center chambermaid" in a summer resort.

"I don't really talk about my dad," says Vic Morrow's daughter. "Too sad." Half-second pause. "In the play there's all that [father/daughter] stuff. What I love about this play is that you can deal with all that stuff, publicly — sort of — in a way a private person can't do." Also in a way a great many other actresses would do less utterly believably.