%I M3747
%S 5,6,7,13,14,15,20,21,22,23,24,28,29,30,31,34,37,38,39,41,45,46,47,52,
%T 53,54,55,56,60,61,62,63,65,69,70,71,77,78,79,80,84,85,86,87,88,92,93,
%U 94,95,96,101,102,103,109,110,111,112,116,117,118,119,120,124,125,126
%N Congruent numbers: positive integers n for which there exists a right triangle having area n and rational sides.
%C Positive integers n such that x^2 + n*y^2 = z^2 and x^2  n*y^2 = t^2 have simultaneous integer solutions. In other words, n is the difference of an arithmetic progression of three rational squares: (t/y)^2, (x/y)^2, (z/y)^2. Values of n corresponding to y=1 (i.e., an arithmetic progression of three integer squares) form A057102.
%C Tunnell shows that if a number is squarefree and congruent, then the ratio of the number of solutions of a pair of equations is 2. If the Birch and SwinnertonDyer conjecture is assumed, then determining whether a squarefree number n is congruent requires counting the solutions to a pair of equations. For odd n, see A072068 and A072069; for even n see A072070 and A072071.
%C If a number n is congruent, there are an infinite number of right triangles having rational sides and area n. All congruent numbers can be obtained by multiplying a primitive congruent number A006991 by a positive square number A000290.
%C Conjectured asymptotics (based on random matrix theory) on p. 453 of Cohen's book. [From _Steven Finch_, Apr 23 2009]
%D R. Alter, The congruent number problem, Amer. Math. Monthly, 87 (1980), 4345.
%D R. Alter and T. B. Curtz, A note on congruent numbers, Math. Comp., 28 (1974), 303305 and 30 (1976), 198.
%D Alter, Ronald; Curtz, Thaddeus B.; Kubota, K. K. Remarks and results on congruent numbers. Proceedings of the Third Southeastern Conference on Combinatorics, Graph Theory and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1972), pp. 2735. Florida Atlantic Univ., Boca Raton, Fla., 1972. MR0349554 (50 #2047)
%D Graeme Brown, The Congruent Number Problem, 2014; http://www.graemebrownart.com/congruent.pdf
%D H. Cohen, Number Theory. I, Tools and Diophantine Equations, SpringerVerlag, 2007, p. 454. [From _Steven Finch_, Apr 23 2009]
%D R. Cuculiere, "Mille ans de chasse aux nombres congruents", in Pour la Science (French edition of 'Scientific American'), No. 7, 1987, pp. 1418.
%D L. E. Dickson, History of the Theory of Numbers, Vol. 2, pp. 459472, AMS Chelsea Pub. Providence RI 1999.
%D R. K. Guy, Unsolved Problems in Number Theory, D27.
%D T. Komatsu, Congruent numbers and continued fractions, Fib. Quart., 50 (2012), 222226.  From _N. J. A. Sloane_, Mar 04 2013
%D G. Kramarz, All congruent numbers less than 2000, Math. Annalen, 273 (1986), 337340.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D J. B. Tunnell, A classical Diophantine problem and modular forms of weight 3/2, Invent. Math., 72 (1983), 323334.
%H T. D. Noe, <a href="/A003273/b003273.txt">Congruent numbers up to 10000; table of n, a(n) for n = 1..5742</a>
%H A. Alvarado and E. H. Goins, <a href="http://arxiv.org/abs/1210.6612">Arithmetic progressions on conic sections</a>, arXiv 2012 [From _Jonathan Sondow_, Oct 25 2012]
%H E. Brown, Three Fermat Trails to Elliptic Curves, <a href="http://www.math.vt.edu/people/brown/doc/ellip.pdf">5. Congruent Numbers and Elliptic Curves (pp 811/17)</a>
%H B. Cipra, <a href="http://sciencenow.sciencemag.org/cgi/content/full/2009/923/3?etoc">Tallying the class of congruent numbers</a>, ScienceNOW, Sep 23 2009
%H Clay Mathematics Institute, <a href="http://www.claymath.org/prizeproblems/birchsd.htm">The Birch and SwinnertonDyer Conjecture</a>
%H Department of Pure Maths., Univ. Sheffield, <a href="http://www.shef.ac.uk/~puremath/theorems/congruent.html">Pythagorean triples and the congruent number problem</a>
%H E. V. Eikenberg, <a href="http://www.math.umd.edu/~eve/cong_num.html">The Congruent Number Problem</a>
%H W. F. Hammond, <a href="http://math.albany.edu:8000/math/pers/hammond/Presen/rsumo.html">A Reading of Karl Rubin's SUMO Slides on Rational Right Triangles and Elliptic Curves</a>
%H Bill Hart, <a href="http://aimath.org/news/congruentnumbers/">A Trillion Triangles</a>, American Institute of Mathematics
%H MathDL, <a href="http://mathdl.maa.org/mathDL/?pa=mathNews&sa=view&newsId=680">Five Mathematicians Capture Record Number of Congruent Numbers</a>
%H Karl Rubin, <a href="http://math.Stanford.EDU/~rubin/lectures/sumo/">Elliptic curves and right triangles</a>
%H W. A. Stein, <a href="http://modular.math.washington.edu/edu/Fall2001/124/lectures/lecture33/html">Introduction to the Congruent Number Problem</a>
%H W. A. Stein, <a href="http://modular.math.washington.edu/simuw06">The Congruent Number Problem</a>
%H Ye Tian, <a href="http://arxiv.org/abs/1210.8231">Congruent Numbers and Heegner Points</a>
%H D. J. Wright, <a href="http://www.math.okstate.edu/~wrightd/4713/nt_essay/node7.html">The Congruent Number Problem</a>
%e 24 is congruent because 24 is the area of the right triangle with sides 6,8,10.
%e 5 is congruent because 5 is the area of the right triangle with sides 3/2, 20/3, 41/6 (although not of any right triangle with integer sides  see A073120).  _Jonathan Sondow_, Oct 04 2013
%t (* The following Mathematica code assumes the truth of the Birch and SwinnertonDyer conjecture and uses the list of primitive congruent numbers produced by the Mathematica code in A006991: *)
%t For[cLst={}; i=1, i<=Length[lst], i++, n=lst[[i]]; j=1; While[n j^2<=maxN, cLst=Union[cLst, {n j^2}]; j++ ]]; cLst
%Y Cf. A057102, A006991, A072068, A072069, A072070, A072071, A073120, A165564, A182429.
%K nonn,nice
%O 1,1
%A _N. J. A. Sloane_.
%E Guy gives a table up to 1000.
%E Edited by _T. D. Noe_, Jun 14 2002
%E Comments revised by _Max Alekseyev_, Nov 15 2008
%E Comment corrected by _Jonathan Sondow_, Oct 10 2013
