I'm reading https://terrytao.wordpress.com/tag/chowlas-conjecture/ and at some point it is mentioned that, the twin prime conjecture is equivaalent to Chowla's conjecture that $\sum_{n\leq x} \lambda(n)\lambda(n+2) =o(x)$, where $\lambda$ denotes the Liouville function. Does anyone have a reference for this result ? Also, what would be de Polignac's conjecture in terms of $\lambda$ ?