When Austin writer Karen Olsson was a teenager, she did a lot of reading. A lot. And not light fiction (though maybe there was some of that) or novels for school (of course), but heavy thinkers from France.

“Yes, while you were at Fugazi shows, I was at home trying to read about Simone Weil,” Olsson, a Washington, D.C., native says to me, somewhat ruefully.

She's speaking of the enigmatic French philosopher whose strange death in 1943 at age 34 obscured her work from the larger public for years. (Simone had an interest in what we might call extreme morality. She died of a combination of tuberculosis and starvation as she refused to eat anything other than what was rationed to French soldiers during World War II, in solidarity with the country's troops. Her death was ruled a suicide.)

Increasingly popular in the decades following her demise, Simone is still a somewhat obscure figure to the general public compared to, say, Sigmund Freud or Albert Camus (who called her “the only great spirit of our times”). But she is now known as an exceedingly complicated visionary.

“As a teenager,” Olsson says, “I think her appeal to me was as much aesthetic as anything else. She was a hardcore French intellectual from French intellectual central casting. She had the glasses and the discipline, and she ate very little and read philosophy.”

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Weil (pronounced like “Vay”) had a brother named André, who lived until he was 92 and mourned his sister his entire life. He was one of the great 21st-century mathematicians, widely respected and honored.

André and his sister were geniuses of the first rank and possessed an intense relationship. It is this relationship that animates and informs Olsson’s “The Weil Conjectures: On Math and the Pursuit of the Unknown.”

The book is part memoir, part biography and part meditation on what it means to be obsessed with math as both a discipline and a source of singular pleasure. Olsson, who majored in math at Harvard, is very much that. However, "The Weil Conjectures” started life as a hunk of fiction, she says.

“I was writing a novel about mathematicians, and it got more unwieldy,” Olsson says.

As she wrote, too many fictional mathematicians were running into too many real mathematicians. It dawned on Olsson that while André was fascinating, he wasn’t working as a character in the novel. Instead of just dumping him, Olsson took a step back and started writing about why she wanted to write about him.

From there, Olsson started writing in a fragmentary way: “I didn't really know what it was. I was just like, man, well, I'm just gonna explore this.”

The results are excellent, some of Olsson’s most evocative writing. For example, an excerpt on her fondness for math as a communal pursuit: “We were a small band of students giddily, exhaustedly trekking through an abstract moonscape, helping one another across patches of ice or fighting over which direction to head next. The egos, the insecurities, the unabashed nerdiness! I miss it still.

"Also, at nineteen, so much is up in the air, open to question, unreliable. I think part of what I liked about math was simply that it seemed like a sure thing, as sure as a thing could be, a solid mass of true and rigorous and irreproachable knowledge that I could grab like a pole on a bus.”

Initially a philosophy major, Olsson says she recognized something crucial about mathematics: College was the best place for it.

“It really felt like, 'This is my one chance,'" she says. “I had a very rosy view of life in that I thought in adulthood I can keep learning and learn German and the history of Jerusalem (for example). But math really needs to be studied with mathematicians in college, if I was going to learn it at all.”

But “Weil Conjectures” is also about André and Simone, older brother with a mind like a jewel and younger sister his intellectual equal who worships her doting but demanding sibling. Being younger, she is not quite at his level and, especially as a teen, frets to the point of suicidal obsession about not being able to keep up. Theirs was a complicated relationship and Olsson was, as the kids say, here for it.

“André was jailed briefly in France in 1940, and the two corresponded back and forth,” Olsson says. “A lot of that correspondence was about math and her trying to understand what he does and kind of pleading with him to explain it to her.”

In her pleading was a question: Could something this complicated be explained to someone who wasn’t fluent in high-level math, and if not, is there really a point to its study?

“This relationship fascinated me in adulthood,” Olsson says, “and the questions become, ‘How did math influence her? How did her brother influence her?’ Even as she is kind of taking shots at his abstraction, she herself is a very abstract thinker, and her way of arguing points and talking about the subjects that she's passionate about can often seem mathematical.

"You can see the kind of kinship of their brains, even though they're such different people.”