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 A247509 #22 Dec 23 2024 14:53:44
%S A247509 3,3,2,4,2,3,2,2,3,2,4,3,2,2,3,3,2,4,3,2,3,2,2,4,3,2,3,2,2,5,2,3,2,4,
%T A247509 2,3,3,2,3,3,2,5,2,3,2,3,5,3,2,2,3,2,3,3,3,3,2,4,3,2,2,5,3,2,2,3,2,4,
%U A247509 2,2,2,3,3,3,2,2,3,2,2,3,2,4,2,3,2,2,4
%N A247509 Number of preprimes (A156759, n>1) such that the smallest prime divisor equals prime(n).
%H A247509 Indranil Ghosh, <a href="/A247509/b247509.txt">Table of n, a(n) for n = 1..1000</a>
%H A247509 Vladimir Shevelev, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2014-September/013643.html">A classification of the positive integers over primes</a>
%F A247509 For n>1, a(n) = pi(prime(n+1)^2/prime(n))-n +1, where pi(x) is the prime counting function (cf. A000720). - _Vladimir Shevelev_, Sep 28 2014
%e A247509 For n=2, using the formula, we have a(2)=pi(25/3)-1=3.
%t A247509 a[1] = 3;a[n_] := PrimePi[Prime[n + 1]^2 / Prime[n]] - n + 1; Table[a[n], {n, 1, 87}] (* _Indranil Ghosh_, Mar 09 2017 *)
%o A247509 (PARI) for (n=1, 87, print1(if(n==1, 3, primepi(prime(n + 1)^2 / prime(n)) - n + 1),", ")) \\ _Indranil Ghosh_, Mar 09 2017
%Y A247509 Cf. A000040, A156759, A247393, A247394.
%K A247509 nonn
%O A247509 1,1
%A A247509 _Vladimir Shevelev_, Sep 18 2014
%E A247509 More terms from _Peter J. C. Moses_, Sep 18 2014