I'm an engineer who works at the intersection of machine learning, mathematics, and large-scale cloud infrastructure. By day I build systems that move recommendations, search, and content understanding for hundreds of millions of users. By night I write the kind of articles I wish I'd had when I was learning these subjects.
This site is a long-form notebook. Every article is meant to give you a complete and honest mental model of one topic — enough that you can read papers, design systems, and recognise the difference between a textbook diagram and what production actually looks like. The series cover recommendation systems, linear algebra, optimisation, NLP, reinforcement learning, ODEs, PDEs, and the cloud infrastructure that ML runs on.
Most articles are written in two languages — English and 中文 — because I want both audiences to get the same depth, not a translated thumbnail of it. The language switch button in the top-right will take you to the paired version of any article.
If you find a mistake, want to argue about a derivation, or just want to say hi: I'm reachable on GitHub or by email at chenkai.nb.666@gmail.com.
Thanks for reading.