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%I #26 Jun 14 2018 02:23:22
%S 6,858,7140,158730,771342,3120180,9699690,31651620,119584290,
%T 198843645,229474245,406816410,281291010,1412220810,1673196525,
%U 3457939485,3234846615,4360010655,4573403835,4127218095,11532931410,12929686770,101268227775
%N Smallest s for which there are exactly n primitive Pythagorean triangles with perimeter 2s; i.e., smallest s such that A078926(s) = n.
%C A Pythagorean triangle is a right triangle whose edge lengths are all integers; such a triangle is 'primitive' if the lengths are relatively prime.
%H Derek J. C. Radden and Peter T. C. Radden, <a href="/A078927/b078927.txt">Table of n, a(n) for n=1..39</a> (terms 1 through 15 were computed by Derek J. C. Radden)
%e a(2)=858; the primitive Pythagorean triangles with edge lengths (364, 627, 725) and (195, 748, 773) both have perimeter 2*858 = 1716.
%t oddpart[n_] := If[OddQ[n], n, oddpart[n/2]]; ct[p_] := Length[Select[Divisors[oddpart[p/2]], p/2<#^2<p&&GCD[ #, p/2/# ]==1&]]; a[n_] := For[s=1, True, s++, If[ct[2s]==n, Return[s]]]
%Y a(n) = A078928(n)/2. Cf. A078926.
%K nonn
%O 1,1
%A _Dean Hickerson_, Dec 15 2002
%E a(8) from _Robert G. Wilson v_, Dec 19 2002
%E a(9)-a(15) from _Derek J C Radden_, Dec 22 2012
%E a(16)-a(23) from _Peter T. C. Radden_, Dec 29 2012
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