It was rare for Louis Leithold to miss a day at Malibu High School, where he taught Advanced Placement calculus for the past several years.
He had been pounding theorems and proofs into the heads of his teenage charges for eight months straight. He had humored them into a homework load -- two hours a night -- that could incite rebellion in most other classrooms. He made them memorize and recite complicated rules of calculus until the theorems ruled their brains. And he moved them with his own mantra, which he recited daily. "We go step by step by step," he would say as he covered all the dry boards in the classroom with equations.
As if that weren't challenging enough, he scheduled two marathon study sessions at his house on Sundays -- the last two Sundays before the Advanced Placement exam May 4. From 9 a.m. to 4 p.m., his 17 students willingly set aside surf, sun and IPods for polynomials and Riemann sums.
"Dr. Leithold," as his students called him, was clearly no ordinary teacher.
First of all, he was 80 years old, a UC Berkeley graduate who earned his PhD in math education long before his students' parents were born. He started teaching high school in his 70s.
But he was revered not only because he had spurned retirement for the rigors of the classroom: He literally wrote the book on his specialty.
He was the author of "The Calculus," a widely used college and high school text praised for its thorough, lucid and logical presentation of one of the most demanding of subjects. Originally published in 1968, it is now in its seventh edition.
He also was a sought-after trainer of calculus teachers. His presence at Advanced Placement seminars could send a wave of excitement through the room.
So when Leithold was found dead at his Pacific Palisades home April 29, the loss was felt not only at Malibu High, where he taught for the last seven years, but across the country.
"Louis is a legend in AP calculus circles," said Trevor Packer, executive director of the AP program for the College Board, the organization that sponsors the exams taken by thousands of high school students every May. The Advanced Placement program offers high school students the opportunity to take college-level courses and earn college credits.
"A lot of his fame is not just due to his textbook," Packer said, "but to his impact on other teachers and students. That's where he left his mark -- in classrooms across the country, through their teachers."
He influenced one of the most famous calculus teachers in America, Jaime Escalante, the former Garfield High School instructor whose success with inner-city students in Los Angeles was told in the 1988 movie "Stand and Deliver."
Escalante, who met Leithold in the 1980s, consulted with him about when to teach certain concepts, such as limits and differentiation. He invited Leithold to lecture to his classes at Garfield and used copies of Leithold's textbook that Leithold helped him obtain at a discount.
"I called him my advisor," Escalante told The Times on Saturday from his son's home near Sacramento. "He was one of the great mathematicians. His book had beautiful problems. It made us believe that anybody could do calculus."
Leithold, who lived alone, failed to show up for class two Fridays ago. His body was discovered by a parent who accompanied three worried students to his house after school that day.
A spokesman for the coroner's office said Leithold had heart and pulmonary disease and attributed the death to natural causes.
His students were distraught at the loss of their teacher -- and not merely because the Advanced Placement exam was just days away.
"With any other teacher who tried to give us as much work, there would have been a class rebellion," said senior Matthew Mesher, 17. "But he inspired you to do mathematics. His face would just light up."
Leithold spent most of his 50-year career in college classrooms. In addition to Cal State L.A., he taught at Phoenix College in Arizona, the Open University of Great Britain, USC and, most recently, Pepperdine University, where he was an adjunct professor of mathematics.
As a boy growing up in San Francisco, he was academically gifted. He attended Lowell High School, an elite public school that accepted only the brightest students in the city. He later worked his way through UC Berkeley, where he earned his bachelor's, master's and doctorate degrees.
He was teaching in Arizona when a publisher approached him about writing a textbook. "The Calculus with Analytic Geometry" was published in 1968 by Harper and Row and quickly became a bestseller in English and several other languages, including German, Spanish and Chinese. In its latest edition, it is called simply "The Calculus 7."