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 A121543 #26 Jul 25 2025 10:18:19
%S A121543 1,3,4,6,8,9,10,11,12,14,15,16,17,18,20,21,22,24,25,26,27,28,30,32,33,
%T A121543 34,35,36,38,39,40,41,42,44,45,46,48,49,50,51,52,54,55,56,57,58,60,62,
%U A121543 63,64,65,66,67,68,69,70,72,74,75,76,77,78,80,81,82,83,84,85,86,87,88
%N A121543 "If k appears then the k-th prime doesn't", with a(1)=1.
%C A121543 Most prime numbers are absent; from first 1000 primes only 138 are terms: 3, 11, 17, 41, 67, 83, 109, 127, 157, 191, 211, 241, 277, 283, 353, ..., all with prime indices. Also, all nonprimes are terms. - _Zak Seidov_, Sep 12 2015
%C A121543 k is a term if and only if A078442(k) is even. - _Pontus von Brömssen_, Jul 25 2025
%H A121543 Zak Seidov, <a href="/A121543/b121543.txt">Table of n, a(n) for n = 1..10000</a>
%t A121543 s={1};Do[If[ !PrimeQ[n]||(PrimeQ[n] && FreeQ[s,PrimePi[n]]),AppendTo[s,n]],{n,2,100}];s (* _Zak Seidov_ *)
%Y A121543 Cf. A078442, A262275.
%K A121543 nonn
%O A121543 1,2
%A A121543 _Zak Seidov_, Aug 06 2006