This article provides a formalized proof of the so-called “the four-square theorem”, namely any natural number can be expressed by a sum of four squares, which was proved by Lagrange in 1770. An informal proof of the theorem can be found in the number theory literature, e.g. in [14], [1] or [23]. This theorem is item #19 from the “Formalizing 100 Theorems” list maintained by Freek Wiedijk at http://www.cs.ru.nl/F.Wiedijk/100/.
@article{bwmeta1.element.doi-10_2478_forma-2014-0012,
author = {Yasushige Watase},
title = {Lagrange's Four-Square Theorem},
journal = {Formalized Mathematics},
volume = {22},
year = {2014},
pages = {105-110},
language = {en},
url = {http://dml.mathdoc.fr/item/bwmeta1.element.doi-10_2478_forma-2014-0012}
}
Yasushige Watase. Lagrange’s Four-Square Theorem. Formalized Mathematics, Tome 22 (2014) pp. 105-110. http://gdmltest.u-ga.fr/item/bwmeta1.element.doi-10_2478_forma-2014-0012/
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