"Oh, it's wonderful! It's my favorite subject!"
     -- Hermione Granger

Arithmancy is a branch of magic which is concerned with the magical properties of numbers. For example, in the 1200s, Bridget Wenlock, a famous Arithmancer, discovered the magical properties of the number seven (fw22).

Arithmancy class at Hogwarts is taught by Professor Vector. In her class, students are expected to write essays and to create and be able to understand complicated number charts (PA12). 

books about Arithmancy
Numerica and Grammatica (textbook used at Hogwarts)