Russian, better; I've been through the Duolingo course many times over now. I like it's teaching style, but it ends too soon, and I'm struggling to find good le

Russian, better; I've been through the Duolingo course many times over now. I like it's teaching style, but it ends too soon, and I'm struggling to find good learning resources to advance to a higher level. More mathematics, particularly more degree-level analysis and geometry. It's the area that interests me most, and I feel I missed out on it doing a computer science degree. I'd also like to understand elliptic curve crypto. Not really a "topic", but I've wanted to get around to learning piano for a while now.