Determinism is true as far as science can tell. By determinism I mean human actions and thoughts are not above natural laws. Given sufficient knowledge of a person's past, he future is as predictable as the future the positions of planet earth.*
Free will is a relation between one's action and his desire, and is thus independent of determinism. If one's action is the outcome of his own desires, then he is free. In other words, whether you will be free or not in the future is as determined as in the past
Physics is not governed by mathematics. Mathematics is the art of saying the same thing in different ways. When a mathematical model contradicts a scientific observation, we revise the mathematical model; we do not punish the physical world for defying the law.
*If I'm not mistaken, Bertrand Russell's view is slightly different from materialism. The difference is similar to that of agnosticism from atheism.