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 A147775 #6 Jan 16 2018 02:46:12
%S A147775 420,840,858,1260,1326,1540,1716,1938,1980,2145,2310,2340,2520,2622,
%T A147775 2652,2730,2850,2860,2964,3080,3450,3465,3740,3876,3960,4002,4080,
%U A147775 4290,4350,4420,4560,4620,4650,4692,4845,4940,5005,5100,5244,5394,5460,5544
%N A147775 Numbers of the form (h+k)k, where h/k is a fraction in a Farey sequence, that occur as at least two times in a Farey sequence. 420 (13+15)*15=(1+20)*20.
%C A147775 This value (h+k)k gives one side of a triangle defined by sides (k^2, (h+k)k, (h+k)^2-k^k) which happens to have the property that the angle opposite the (h+k)k side is twice the measure of the angle opposite the k^2 side. These numbers would be the duplicates in the term-by-term multiplication: A106501 * A106500.
%H A147775 N. M. Makin, <a href="/A147775/b147775.txt">Table of n, a(n) for n = 1..5490</a>.
%e A147775 a(n) <- (h1,k1),(h2,k2)
%e A147775 420 <- (13,15),(1,20)
%e A147775 840 <- (19,21),(11,24)
%e A147775 858 <- (17,22),(7,26)
%e A147775 2160 <- (17,28),(1,35)
%e A147775 2520 <- (23,40),(11,45)
%t A147775 F[n_]:= Union[Flatten[Join[Table[p/q, {q, n}, {p, q - 1}]]]];
%t A147775 S[n_]:=(Denominator[n]+Numerator[n])Denominator[n];
%t A147775 FindDups[l_]:= Module[{sl,rs},sl=Sort[l];rs={};Map[If[Count[sl,# ]>1,rs=Append[rs,# ]]&,l]; Union[rs]];
%t A147775 FindDups[S[F[71]]]
%Y A147775 Cf. A007305, A007306, A106500, A106501.
%K A147775 nonn
%O A147775 1,1
%A A147775 Nicholas M. Makin (nmaximillian(AT)yahoo.com), Nov 12 2008