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 A007237 Number of triangles with integer sides and area = n times perimeter. (Formerly M3878) 7
 5, 18, 45, 45, 52, 139, 80, 89, 184, 145, 103, 312, 96, 225, 379, 169, 116, 498, 123, 328, 560, 280, 134, 592, 228, 271, 452, 510, 134, 1036, 144, 280, 639, 339, 597, 1119, 139, 354, 635, 648, 162, 1486, 169, 594, 1215, 354, 186, 1066, 369, 622 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Ray Chandler, Table of n, a(n) for n = 1..5000 (first 300 terms from Zhining Yang) Juan V. Savall and Jesus Ferrer, Problem E3408, Amer. Math. Monthly, 99 (1992), 175-176. FORMULA a(n) = A120062(2n). - Ray Chandler, Jul 27 2017 EXAMPLE For n=2, the a(2)=18 solutions whose area is twice its perimeter: (13,14,15) (12,16,20) (15,15,24) (10,24,26) (11,25,30) (18,20,34) (15,26,37) (14,30,40) (10,35,39) (9,40,41) (12,50,58) (33,34,65) (25,51,74) (9,75,78) (11,90,97) (21,85,104) (19,153,170) (18,289,305). PROG (PARI) for(k=1, 100, n=0; d=4*k^2; e=3*d; for(b=1, sqrt(e), for(c=2*k, e/b, if(b*c>d && c>=b, f = (b + c)*d / (b * c - d); if(f>=c, a=floor(f); if(a==f, n++))))); print1(n, ", ")) CROSSREFS Sequence in context: A101105 A037140 A321049 * A000339 A270944 A272457 Adjacent sequences:  A007234 A007235 A007236 * A007238 A007239 A007240 KEYWORD nonn AUTHOR EXTENSIONS a(16)-a(50) from Les Reid, Jul 06 2010 STATUS approved

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Last modified February 19 23:27 EST 2019. Contains 320329 sequences. (Running on oeis4.)