RUMORS TO THE CONTRARY notwithstanding, Tom Lehrer is not now and never has been dead. Of course, he's heard the talk and seen published reports of his demise more than a few times. He even keeps a "Dead" file in his desk--with the London newspaper that praised a new revue of songs "written by the late Tom Lehrer," and the German clipping that announced his demise from "a loathesome disease." "I was hoping the rumors would cut down on the junk mail," Lerher said recently, "but they didn't."

His name, as the saying goes, rings a bell. It sounds vaguely sixties-ish, though not really political. It also has a certain show biz resonance, but that's not exactly right either. Even at the height of his fame--from the mid '50s to the mid '60s--he occupied an unusual niche, and now, with the passage of time, Tom Lehrer has fallen between the cracks of celebrity.

That is what he prefers. He has not written many songs since 1965 and has not performed regularly in the United States since 1960. His sporadic, 20-year quest for a Harvard Ph.D. in Mathematics ended in 1965. During the last decade he has lived half each year in Cambridge and spent the winter months as a visiting teacher at the University of California at Santa Cruz. "When you're in a public profession like I was, and you stop doing it like I did," Lehrer says, "people think you're eighter crazy or dead." Tom Lehrer, 53 years old and voluntarily retired from the public eye, is neither.

He was born in 1928 and raised on the upper East Side of New York City, where he spent an unremarkable childhood--piano lessons and private school. After graduating from Horace Mann high school, he arrived at Harvard simultaneously with World War II. "I thought about majoring in Math, Chemistry and English," Lehrer says, "but Math had the fewest requirements so I went with it." After graduating magna cum laude in 1946, he continued studying mathematics in Harvard graduate school. "I knew I wanted to teach and Math was my field, so I studied Math," he says.

While at Harvard, Lehrer began fiddling with a piano in Dunster House, writing ditties about campus personalities, parodying popular songs of the day, and playing his compositions at parties. He thought little of it at the time; it was just a way to entertain himself and his friends.

Lehrer began his long pursuit of a Ph.D. in 1946. In 1951, "fate intervened," Lehrer says mock-portentously, when he and three other grad students entered a quartet contest on Arbor Day at the Law School. They were, it turns out, the only entry, so the sponsors withdrew the prize, but the group evoked such a favorable response that they decided to keep singing. At least Lehrer did. "The other guys went on to bigger and better and more respectable things--one's now at the Carnegie Foundation and the other two are professors at Tulane and Case Western. They all scattered, and I was left holding the repertoire."

His first important gig came at the Freshman Smoker, a bawdy, all-male (and long extinct) annual revue that Harvard's freshman class used to sponsor. Lehrer played the Smoker for four straight years, parlaying his original songs, parodies and piano-playing into a small-time institution. He decided to make a record.

"If I couldn't have made the record," Lehrer says, "I would have stopped singing. But the 1.p. had just been invented and mine sold okay. Then the students took them home for the summer, and, like herpes, they spread. I began getting mail orders."

IN 1953, SEVEN YEARS into his graduate studies, Lehrer decided to give show business a full-time try. He started at the Blue Angel nightclub in New York and played clubs and talk shows from coast to coast for a few years. He cut four albums, which eventually sold a total of 1.5 million copies. In 1960, at the height of his fame, he stopped performing regularly to continue his quest for the elusive Ph.D.

The situation, Lehrer recalls, was not at all peculiar. "I just studied and was a section leader and nothing was strange about it at all. I was never really confronted with my difference from other grad students. It wasn't like I was Brooke Shields coming to entertain the troops." One Harvard professor, for whom Lehrer was a teaching fellow, remembers Lehrer this way. "He was Tom Lehrer, world celebrity, the most famous member of the Math department," John Tate, Perkins Professor of Mathematics, says, "but on the other hand, when he was here, that part of him wasn't at all obvious." Lehrer kept after the doctorate for five years under the supervision of Frederick Mosteller, professor of mathematical statistics. ("By the end, I think he would have written it for me, he wanted to see me do it so badly," Lehrer says of his former mentor), but his dissertation just never came together. "In 1965 I realized that I didn't want [the Ph.D.] so badly, so I stopped working for it," Lehrer says. That was also the year he cut his last album--music that accompanied a short-lived television program called "That Was; the Week That Was," starring Buck Henry. With that, he; retired from show business to teach "mathematical models in social science" at MIT until 1971.

"I stopped performing because I don't have the temperament of a performer. You have to want to do the same thing over and over again. Once I got it right, I didn't want to do it again. I always use the analogy of a novelist who has to read his novel in public night after night. I just didn't want to do it," Lehrer says. "And also," he adds in an obviously well-rehearsed phrase, "I had no more desire for anonymous affection." Those reasons, plus a distaste for travel ("Once you've been to Detroit there's no point in going to Cincinnati") prompted him to quit cold turkey. He says he has never felt tempted to return to the stage.

WHEN LEHRER broadened his horizons in the '50s, he did so with a form of music that has virtually disappeared today. His appeal was not really that of a musician, though he had (has?) a pleasing, friendly voice and a knack for a catchy tune. Nor was he a full-fledged comedian, for music dominated his act. He didn't exactly grind a political axe, either, but always cast a bemused, skeptical eye on current events for most of his material. Lehrer calls his technique "gentle making fun of things from a point of view of caring about them." His albums today reveal an abundance of that gentle spirit. It is easy to see why his popularity with the college set faded dramatically in the late '60s, a time in which good-humored tolerance was, in the words of the time, not relevant.

His favorite subjects included nuclear war ("Who's Next?" about the arms race, and "We Will All Go Together When We Go,") personal degeneracy ("SMUT" and "The Masochism Tango") and human ineptitude in all its inglorious splendor. At the root of all Lehrer humor seemed to be the assumption that things could get better if only people stopped screwing up. He looked not for malevolence but rather for benign stupidity--not for evil but for incompetence.

Lehrer attributes his fall from favor in the late '60s to a sudden inappropriateness "of my kind of humor." Using the tools of his mathematical trade, he calls his appeal a "bi-modal distribution." "The people who were in college in the '50s were my first real audience and their kids, the people who found my records in the cabinet during their Mad-magazine years picked me up also," Lehrer says. The group in between, who were in college during "that less humorous time," never accepted Lehrer's brand of humor.

"I think my stuff is more suitable for 1981 than it was in 1971," Lehrer says and, in keeping with that assumption, he has seen a recent mini-renaissance of his work. A revue of his songs called Tomfoolery played London for a year ("Every song-writer in the world has had a revue and they finally got to the bottom of the barrel," he says) and the show, with its cast of four, is coming to New York in December. He also has published a new collection of his songs called Too Many Songs by Tom Lehrer (Pantheon; $16.50), with improved piano arrangements and two songs that he wrote for public television's Electric Company.

But for all the renewed interest, Lehrer maintains steadfastly that nothing will draw him back to a public piano. He is happy, he says, spending January to June as a teacher at Santa Cruz, where the snow never falls, and living in Cambridge for the rest of the year. A piano dominates the book-lined living room of his Cambridge house, but he says the temptation to lampoon our current national and personal follies never overcomes him. It's easy to get the impression that things may just not be as funny as they were in Lehrer's prime. Perhaps the combination of ego and bluster to which Lehrer attributed most of the world's ills no longer suffices as an explanation, even in comedy; perhaps humor today--from Mork to Garp--must live on its own, away from the world, because the world isn't very funny. Tom Lehrer, on the other hand, thought people could think and laugh at the same time. He says he still thinks so, but he's teaching math in California now.