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 A020890 #15 May 06 2021 01:50:42 %S A020890 3,10,12,15,21,30,35,36,40,44,52,55,56,63,65,70,77,78,85,90,99,102, %T A020890 105,114,117,119,126,132,133,136,143,152,154,161,165,168,171,176,182, %U A020890 184,187,195,198,200,207,208,210,216,221,225,230,234,240,247,253,255,260,261,270 %N A020890 Ordered set of (b + c - a)/2 as (a,b,c) runs through all primitive Pythagorean triples with a < b < c. %C A020890 From _Bernard Schott_, May 06 2021: (Start) %C A020890 Also, ordered sides b of primitive triples (a, b, c) for integer-sided triangles where side a is the harmonic mean of the 2 other sides b and c, i.e., 2/a = 1/b + 1/c with b < a < c (A343892). %C A020890 The first term appearing twice is 330 = a(71) = a(72). (End) %H A020890 Ray Chandler, <a href="/A020890/b020890.txt">Table of n, a(n) for n = 1..10000</a> %F A020890 a(n) = A020889(n)/2. %Y A020890 Cf. A020889. %Y A020890 Cf. Triangles with 2/a = 1/b + 1/c: A343891 (triples), A020883 (side a), A343892 (side b), A343893 (side c), A343894 (perimeter). %K A020890 nonn %O A020890 1,1 %A A020890 _Clark Kimberling_ %E A020890 Offset corrected to 1 by _Ray Chandler_, Jan 23 2020