Self-published (to keep the cost low) through KDP Print (2018), 200 pages.
Intended audience: Advanced high school and beyond.
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Kindle eBook (print replica): $5:95
This book is a collection of 57 very difficult math problems (with detailed solutions, along with many extensions and variations) that I have compiled over the years. Many of the problems appeared on my old Problem of the Week webpage. I finally (re)polished up a number of those, added some new ones, and created this book. The problems are mostly classics that have been around for ages. They are divided into four categories: General, Geometry, Probability, and Foundational. (Cover illustration: Maki Naro)