@MISC{LAGRA4SQ.MIZ,
  AUTHOR = {Watase, Yasushige},
  TITLE = {Lagrange's Four-Square Theorem},
  SECTION1 = {Preliminaries},
  SECTION2 = {Any Prime Number Can Be Expressed as the
    Sum of Four Integer Squares},
  SECTION3 = {Proof of Lagrange's theorem},
  DAY = {4},
  MONTH = {June},
  YEAR = {2014},
  ADDRESS1 = {Suginami-ku Matsunoki 6, 3-21 Tokyo\\ Japan},
  SUMMARY = {This article provides a formalized proof of
  the so-called "the four-square theorem",
  namely any natural number can be expressed by the sum
  of four integer squares, which was proved by Lagrange in 1770.
  A written proof of the theorem can be found in a book
  on number theory
  such as \cite{HardyWright} or \cite{Baker:1984}.
  \par
  The theorem is listed \#19 on Freek Wiedijk's "Top 100 mathematical
  theorems" list.}}

@BOOK{HardyWright:2008,
 AUTHOR={Godfey H. Hardy, Edward M. Wright},
 TITLE={An Introduction to The Theory of Numbers 6th edition},
 PUBLISHER={Oxford University Press},
 YEAR=1979}

@BOOK{Baker:1984,
 AUTHOR={Baker, Alan},
 TITLE={A Concise Introduction to the Theory of Numbers},
 PUBLISHER={Cambridge University Press},
 YEAR=1984}
