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 A225487 #9 Apr 08 2016 07:34:20
%S A225487 3,5,11,7,13,101,47,53,23,19,29,37,31,41,83,73,17,43,67,157,179,167,
%T A225487 79,443,139,113,137,97,233,61,823,71,103,151,199,499,181,229,353,313,
%U A225487 1889,271,317,197,613,607,127,257,89,367,223,433,239,911,109,107,557
%N A225487 Duplicate primes found by Rowland's recurrence in the order of their reappearance.
%C A225487 Among the first 10^8 terms of A132199 (Rowland's sequence of 1s and primes), 121 terms are prime. Eleven of them appear more than once, and so are a(1), ..., a(11).
%C A225487 Among the first 10^100 terms of A132199 there are 18321 primes; of these, 3074 are distinct and 351 repeated. - _Giovanni Resta_, Apr 08 2016
%C A225487 See the crossrefs for references, links, and additional comments.
%H A225487 Giovanni Resta, <a href="/A225487/b225487.txt">Table of n, a(n) for n = 1..351</a>
%e A225487 The first duplicate in Rowland's sequence of primes A137613 = 5, 3, 11, 3, 23, 3, 47, 3, 5, ... is 3, so a(1) = 3. The second duplicate is 5, so a(2) = 5.
%t A225487 t = {}; b1 = 7; Do[b0 = b1; b1 = b0 + GCD[n, b0]; d = b1 - b0; If[d > 1, AppendTo[t, d]], {n, 2, 10^8}]; L = {}; Do[ If[MemberQ[Take[t, n - 1], t[[n]]], AppendTo[L, t[[n]]]], {n, 2, Length[t]}]; DeleteDuplicates[L]
%Y A225487 Cf. A132199, A137613, A221869.
%K A225487 nonn
%O A225487 1,1
%A A225487 _Jonathan Sondow_, May 08 2013
%E A225487 a(12)-a(57) from _Giovanni Resta_, Apr 08 2016