Marie Crous was a French mathematician active in the 17th century, though very little is known about her life, including her exact birth and death dates. She worked as a tutor for a young noblewoman and was supported by a wealthy patroness connected to Cardinal Richelieu’s family. Crous is known for two books on arithmetic: one from 1636 that explained decimal fractions, and one from 1641 that gathered practical arithmetic rules. She proposed introducing the decimal system in France as early as 1636, years before it was more widely adopted.
