Zero sum game

Cas Russell is good at math. Scary good. The vector calculus blazing through her head lets her smash through armed men twice her size and dodge every bullet in a gunfight, and she'll take any job for the right price. As far as Cas knows, she's the only person around with a superpower...until she discovers someone with a power even more dangerous than her own. Someone who can reach directly into people's minds and twist their brains into Moebius strips. Someone intent on becoming the world's puppet master. Cas should run, like she usually does, but for once she's involved. There's only one…