About

1. What this site is for

Textbooks omit what is obvious to the author — which is exactly what the reader needs. Rikai follows three rules.

• Build from definitions. Every symbol and term is defined in the article before it is used.
• Fill in the proofs. Each step states which hypothesis and which earlier result it uses.
• Say why it exists. Every theorem comes with what it is for and where it is used.

2. Reading with an LLM

• “Open in Claude” / “Open in ChatGPT” starts a conversation with the article body attached. It runs in your own account; this site never receives the content.
• “Copy Markdown” copies the whole article; hovering a formula and pressing LaTeX copies that formula’s source.
• Raw Markdown for every article is served at /raw/<article-id>.md, and the site index for LLMs is at llms.txt.

3. Free and paid

The first two or three chapters of every theme are free in full — never truncated mid-argument. The remaining chapters are collected into a book per theme.

4. Who writes it

Written and reviewed by a software engineer with a PhD in mathematics, with experience teaching at university and writing textbooks. Operated by Mugen Giken LLC (Minato-ku, Tokyo). We also take on contract development of AI and mathematical systems — see contact.