# About

> Rikai is a textbook site for mathematics, physics and computer science, written so that the gaps between the lines are filled in. Operated by Mugen Giken LLC, Tokyo.
> https://rikai.mugen-giken.com/en/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](/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](/contact).