Daniel Sank

Python has type annotations! Adding them would enable editors to identify mistakes and would allow static typechecking as a test (i.e. via [mypy](http://mypy-lang.org/index.html)).

Now that we're getting pull requests, we should have a contributor's section listing those who have contributed.

Options: * Write the problem statement directly into the main document. This seems like a good idea in any case, as it lets people who don't have the book view...

See [Math Stack Exchange post](http://math.stackexchange.com/questions/2205058/prove-linear-relation-between-random-variables-based-on-correlationc).