Equa! core book
Equa! is a minimalist esoteric OSR game. It gives you the tools to play but leaves how to use those tools up to the user.
The Equa! core book contains the Equa! rules themselves, a small bestiary to get you started having adventures and a dungeon to adventure in. All of the content is presented to the user as a set of mathematical equations that are only as correct as needed to be able to facilitate the playing of the game.