“This is Riemann’s zeta function,” she said. It was the one we’d seen that morning on John Doe’s back. “It’s theoretical mathematics. Does this really have something to do with one of your cases?”

Sampson nodded. “Without going into too much detail, we’re wondering why this might be on someone’s mind. Maybe obsessively.”

“It’s on a lot of people’s minds, including mine,” she said. “Zeta’s the core of Riemann’s hypothesis, which is arguably the biggest unsolved problem in mathematics today. In the year two thousand, the Clay Institute offered a million dollars to anyone who could prove it.”

“Sorry, prove what?” I said. “You’re talking to a couple of high school algebra cutups here.”

Sara sat up straighter, getting into it now. “Basically, it’s about describing the frequency and distribution of all prime numbers to infinity, which is why it’s so difficult. The hypothesis has been checked against the first one and a half billion instances, but then you have to ask yourself — what’s one and a half billion compared to infinity?”

“Exactly what I was about to ask myself,” Sampson said, straight-faced.

Sara laughed. She looked almost exactly the same as she did back when we were all pooling our pocket change for pitchers of beer. The same quick smile, the same long hair flowing down her back.

“How about the other two sets of numbers?” I asked. These were the ones that had been carved into the victims’ foreheads.

Sara glanced down for a second, then turned to her laptop and googled them from memory.

“Yeah, right here. I thought so. Mersenne forty-two and forty-three. Two of the biggest known prime numbers to date.”

I scribbled some of this down while she spoke, not even sure what I was writing. “Okay, next question,” I said. “So what?”

“So what?”

“Let’s say Riemann’s hypothesis gets proved. What happens then? Why does anyone care?”

Sara weighed the questions before she answered. “There’s two things, I suppose. Certainly, there are some practical applications. Encryption could be revolutionized with something like this. Writing and breaking code would be a whole new game, so whoever you’re chasing might have that in mind.”

“And number two?” I asked.

She shrugged. “The whole because-it’s-there aspect. It’s a theoretical Mount Everest — the difference being that people have actually been to the top of Everest. Nobody’s ever done this before. Riemann himself had a nervous breakdown, and that guy John Nash from A Beautiful Mind? He was obsessed with it.”

Sara leaned forward in her chair and held up the page of numbers so we could see them. “Let’s put it this way,” she said. “If you’re looking for something that could really drive a mathematician crazy, this is as good a place to start as any. Are you, Alex? Looking for a crazy mathematician?”

Chapter 59