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 A087461 #16 Jan 17 2020 10:38:52
%S A087461 5,9,13,20,25,30,36,42,50,55,63,71,75,80,92,99,106,115,122,127,136,
%T A087461 141,156,163,170,177,185,190,197,210,221,227,238,249,255,265,273,282,
%U A087461 291,300,307,317,325,330,339,351,363,374,385,395,404,409,422,429,438,444
%N A087461 Arithmetic mean of n-th and 2n-th primes.
%C A087461 A rather sparse subset are primes: 5, 13, 71, = a(2), a(4), a(13).
%H A087461 Seiichi Manyama, <a href="/A087461/b087461.txt">Table of n, a(n) for n = 2..10000</a>
%F A087461 a(n) = (n-th prime + 2n-th prime)/2
%e A087461 Series begins with 2nd prime, 3 and 4th prime, 7. Then (3 + 7)/2 = 5.
%t A087461 Table[Mean[{Prime[n],Prime[2n]}],{n,2,60}] (* _Harvey P. Dale_, May 25 2011 *)
%Y A087461 Cf. A000040, A092390, A258934.
%K A087461 nonn,easy
%O A087461 2,1
%A A087461 _Gary W. Adamson_, Sep 07 2003
%E A087461 More terms from _Ray Chandler_ and _Rick L. Shepherd_, Sep 09 2003