Albert Girard (1595, Saint-Mihiel − 8 December 1632, Leiden) was a French-born mathematician. He studied at the University of Leiden. He "had early thoughts on the fundamental theorem of algebra" and gave the inductive definition for the Fibonacci numbers. He was the first to use the abbreviations 'sin', 'cos' and 'tan' for the trigonometric functions in a treatise. Girard was the first to state, in 1632, that each prime of the form 1 mod 4 was the sum of two squares. (See Fermat's theorem on sums of two squares.)
...the first person who understood the general doctrine of the formation of the coefficients of the powers from the sum of the roots and their products. He was the first who discovered the rules for summing the powers of the roots of any equation.
In his paper, Funkhouser locates the work of Girard in the history of the study of equations using symmetric functions. In his work on the theory of equations, Lagrange cited Girard. Still later, in the nineteenth century, this work eventuated in the creation of group theory by Cauchy, Galois and others.
Girard also showed how the area of a spherical triangle depends on its interior angles. The result is called Girard's theorem.
- O'Connor, John J.; Robertson, Edmund F., "Albert Girard", MacTutor History of Mathematics archive, University of St Andrews.
- Dickson, Leonard Eugene (1919). "Ch. XVII: Recurring series; Lucas' un, vn". History of the Theory of Numbers, Vol. I: Divisibility and Primality. p. 393.
- Dickson, Leonard Eugene (1920). "Ch. VI: Sum of two squares". History of the Theory of Numbers, Vol. II: Diophantine Analysis. pp. 227–228.
- Funkhouser, H. Gray (1930). "A short account of the history of symmetric functions of roots of equations". Amer. Math. Monthly 37 (7): 357–365. doi:10.2307/2299273. JFM 56.0005.02.
- The Galileo Project: Girard, Albert