This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A386981 #7 Aug 19 2025 22:27:57 %S A386981 0,3,9,14,12,35,21,39,44,44,23,124,28,73,97,81,30,166,31,130,169,95, %T A386981 39,283,59,90,131,208,33,347,43,160,196,109,160,466,35,117,197,304,41, %U A386981 515,57,267,354,127,61,550,110,214,219,258,44,425,215,484,265,128,51,977,41,138,582,269,169,603,48,325,252,564,47,1058,65,133,445,341 %N A386981 Number of obtuse Heronian triangles with integer inradius n. %C A386981 If a Heronian triangle has an inradius n, and sides (x, y, z), where x <= y <= z, then the triangle is obtuse iff n > (x+y-z)/2. %C A386981 The only Heronian triangle with inradius 1 is the right triangle (3, 4, 5). %C A386981 The number of right integer triangles with inradius n is given by A078644, the number of acute Heronian triangles with inradius n is given by A386980 and the total number of Heronian triangles with inradius n is given by A120062. %H A386981 Alan F. Beardon and Paul Stephenson, <a href="https://www.researchgate.net/publication/304216979_The_Heron_parameters_of_a_triangle">The Heron parameters of a triangle</a>, Mathematical Gazette May 8, 2014. %H A386981 Frank M Jackson, <a href="/A386981/a386981.txt">Mathematica program</a> %e A386981 a(2) = 3, and the 3 obtuse Heronian triangles with inradius 2 have sides (6, 25, 29), (7, 15, 20), (9, 10, 17). %t A386981 (* See link above. *) %Y A386981 Cf. A078644, A120062, A386980 %K A386981 nonn,new %O A386981 1,2 %A A386981 _Frank M Jackson_, Aug 11 2025