The event was advertised as a "Funny Numbers" conversation between comedian Steve Martin and mathematics scholar Robert Osserman . They were going to chat about equations and film and writing, in a cafe-style setting on the stage of the Herbst Theatre.

But when it was over, the audience was amazed at what else they'd witnessed:

Martin taking out a banjo and playing the instrument like a country music star; Osserman saying some things as wittily as his famous guest; and -- halfway through -- Robin Williams suddenly walking onstage, where he bantered, imitated, cajoled and gesticulated (about math, Trent Lott, French people, etc. ) as only Robin Williams can do.

It was a mostly unscripted exercise in comic madness -- all because of math.

Or, rather, in spite of it. Berkeley's Mathematical Sciences Research Institute, where Osserman works, hosted the Sunday night talk (in conjunction with City Arts & Lectures) to raise its profile and public awareness about the role of math in society. Martin played along but digressed every chance he got -- much to the capacity crowd's delight .

"In high school, I used to be able to make magic squares, " Martin said, referring to the patterns of numbers in a square that add up the same in all directions. "I like anything kind of 'jumbly.' I like anagrams. What else do I like? I like sex."

Martin really does like math, and he's been friends with Osserman for about a year, after meeting him at a philosophical society. A professor emeritus at Stanford, Osserman helped Martin flesh out a math-oriented character for his new novella, "The Pleasure of My Company," which is being published next spring.

"The character is slightly neurotic, has compulsions and is struck by anxiety, and one of the ways he calms down is to go home an d create a magic square," Martin said before reading a passage from it: "Making a magic square would alphabetize my brain. Alphabetize is my slang for 'alpha-beta-ize,' meaning raise my alphas and lower my betas. . . . During moments of crisis, I've created magic squares composed of 8, 1 6 and even 64 boxes, and never once has it failed to level me out. . . . Benjamin Franklin was a magic square enthusiast. I assume he tackled them when he was not preoccupied with bopping a Parisian beauty -- a practice I do not have."

So, in Martin's world, math and sex are frequently related in thought. Same with Williams, who at one point pretended to be a lascivious math geek in a boudoir, saying, "Yeah, baby. I'm getting ready to bisect that angle! . . . Oh,

obtuse! . . . Come on and show me that diameter!"

The crowd howled with laughter. So did Osserman, whose be st lines were explaining how banjo frets have the exact same specificati ons as slide rules.

"Slides rules and (the frets of) banjos are divided into even parts where each integral is a fixed fraction," Osserman said, "so if you want to multiply two numbers and you don't have your slide rule or calculator, you bring out two banjos, slide them next to each other, and multiply."

"This," offered Martin, "would be so much more convenient ."

Martin, who shared a Grammy Award for his banjo playing on Earl Scruggs' "Foggy Mountain Breakdown," has been playing the instrument for more than 20 years. He practically got a standing ovation for his brief song on Sunday night, but it was when he called Williams to the stage that comic mayhem broke out.

Among their best moments:

-- Williams reminding people that Europe had no concept of zero before going to the Middle East for the Crusades and appropriating Arabic numbers. "Up to the Crusades, the number zero didn't exist!" Williams bellowed. "Zero was a primal concept. (From then on), you could go negative!"

-- Martin playing Pablo Picasso and Williams playing Albert Einstein as they read from Martin's hit play, "Picasso at the Lapin Agile" -- only Williams read the lines as if he were Jack Benny, Marlon Brando, Peter Lorre, Elmer Fudd and other notable figures.

-- Martin, asked how he would explain the big bang theory to a 5-year-old, saying, "Well, (I'd say) 'There was nothing and suddenly there was everything. Now go wash up.' "

-- Toward the end, Martin chimed in with Williams, who was dominating the stage, saying sarcastically, "Sorry to interrupt." "It's your night," Williams said. "Was," replied Martin.

-- Williams getting handed a white towel for his sweat, putting it over his right hand and pretending he was Michael Jackson dangling a baby over a hotel balcony, then that the towel covered up an Afghan ventriloquist's doll.

When it was over, there was a buzz as people walked out of Herbst Theatre --

the sort of excitement that happens when people believe they've witnessed brilliance. The night was brilliant. The audience got its money's worth (tickets were $21), and Martin, Williams and Osserman were all smiles as they left the stage, knowing they had put on a good show that was worthy of Broadway. The three men had proved a maxim of sorts: Numbers by themselves aren't funny; it's the people using those numbers who can bring real humor to a subject that -- for many people -- is dull and tedious.

"It was wild and all over the place," said David Eisenbud, director of the Mathematical Sciences Research Institute, in something of an understatement. "I'm very pleased with the way things went."

E-mail Jonathan Curiel at jcuriel@sfchronicle.com.