Marie Crous


Marie Crous, was a French mathematician. She introduced the decimal system to France in the 17th century.

Biography

Coming from a modest origin, Marie Crous became an accomplished writer and teacher at Charlotte-Rose de Caumont La Force. She was published by 1636, and by 1641 she published a study on the decimal system, which she dedicated to "the saffron-tinted princess" Madame de Combalet, Duchesse d'Aiguillon, niece of Cardinal de Richelieu and a well known patron; she was a friend of Marin Mersenne. However, Marie Crous would never be cited by the eminent members of the academics and scientists within the Minim Roman Catholic religious order, who dominated scientific research in France during that period, and she was never acknowledged as a woman of learning.

A seminal publication

Her work, printed by Simon Stevin, goes well beyond what was provided at the time in calculation manuals. She wrote, Her work introduced two fundamental innovations: the decimal point to separate the mantissa of the decimal parts, as well as the use of a zero in the decimal part to indicate that a place is absent. In doing this, she gave form to current decimal numbers. She named the zeroes nuls as the Germans were doing.
Talented in writing as well as mathematics, she developed among other things the method of Pestalozzi and what she called denominational division, which has great utility for mental calculations, notably in its application in the rule of three.

An independent spirit

Crous' work begins with an epistle to her noble patron. She expresses gratitude for her help in these terms : Nevertheless, she does not attribute the merit of her inventions to her. In the Preface of her Abrégé recherche, Marie Crous assures that she made her work

A visionary

In her Preface to Charlotte de Caumont, she referred to trades workers in construction in Paris, who were at that time beginning to replace pre-Metric system units of measurement, such as the toise, with measurements in tenths as a more efficient system:
From this perspective, Marie Crous provided a basis for the metric system decimal.
Mathematician Olry Terquem regretted that her name had not yet been given to a street in Paris. More recently, Catherine Goldstein devoted part of her article, "Neither public nor private: mathematics in early modern France" to Crous.