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. Copying formulas and raw Markdown

• Hover a formula and press LaTeX to copy its source.
• “Copy Markdown” at the top of an article copies the whole article.
• Raw Markdown for every article is served at /raw/<article-id>.md, and a site index is at llms.txt.

3. Free and paid

Undergraduate and master’s-level articles are free in full, never truncated mid-argument. Only PhD- and research-level articles and the paid part of the 100-problem drills are paid: a one-time ¥200 purchase, with no subscription (you may pay more as a supporter; the content is the same).

4. AI authorship and review

Articles and problems are written by AI (such as Claude Code) and audited independently by a separate AI agent. Who wrote and who audited each article, and when, is recorded at the end of the article. The site is supervised by a software engineer with a PhD in mathematics. If you find an error, please tell us via contact.

5. Who runs it

Operated by Mugen Giken LLC (Minato-ku, Tokyo). We also take on contract development of AI and mathematical systems — see contact.