Irish mathematician Tom Petrie is called in as an expert to analyze a mysterious stream of particles that appears to be a message from aliens. The math never gets very deep. Petrie is supposed to be an expert on chaos theory, but here that means little more than "making sense out of a mess":
(quoted from The Lure)
"I'm a mathematician. Nobody understands me, I work on ferociously specialised stuff."
"What sort of ferociously specialised stuff?"
"I supposed you'd call it pattern recognition. At the moment I'm doing knots."
"You mean like in string?"
"Yes, only I do them in fourdimensional space."
"I can't visualise that. No wonder nobody understands you. Anyway, it sounds useless."
"Don't you believe it. I've found links with quantum theory and cryptography." He patted his canvas bag as if it contained the secrets of the Universe...

(Really, chaos theory is the study of dynamical systems that are sensitive to initial conditions and transitive.) However, a brief passage at the beginning where Petrie reads through his email (printing out an article on a symplectic approach to chaos and deleting a message claiming to have a proof of Goldbach's conjecture) and a page near the end about how he developed an interest in mathematics in the first place are possibly interesting to fans of mathematical fiction. In the end, the numbers one through four are involved in his final explanation of what it is, and there are also some icosahedra involved.
Forgetting about the math for a moment, The Lure is an SF adventure that combines viruses, aliens, the Manhattan Project and the apparent "fine tuning" of the laws of physics to support life together with a bunch of stereotypes (of mathematicians, Russians, Scandanavian women, evil government conspiracies, etc.) into an uneven but sufficiently entertaining tale. 