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 A247394 #36 Feb 26 2025 05:54:06 %S A247394 26,28,30,32,34,36,38,40,42,44,46,48,51,57,63,69,75,81,87,93,99,105, %T A247394 111,117,125,145,155,203,217,259,287,319,341,377,403,481,629,697,731, %U A247394 799,893,989,1081,1219,1357,1537,1829,1961,2183,2419,2501,2747,2881,3053 %N A247394 Numbers n for which the third maximal prime <= sqrt(n) is the least prime divisor of n. %C A247394 These numbers we call "preprimes" of the third kind in contrast to A156759 for n>=2, for which the maximal prime <= sqrt(n) is the least prime divisor of n; and to A247393 for which the second maximal prime <= sqrt(n) is the least prime divisor of n. %H A247394 Jens Kruse Andersen, <a href="/A247394/b247394.txt">Table of n, a(n) for n = 1..10000</a> %H A247394 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 A247394 lpf(a(n)) = prime(pi(isqrt(a(n)))-2), with pi(n) = A000720(n), lpf(n) = A020639(n) and isqrt(n) = A000196(n). %t A247394 Select[Range[4000], Prime[PrimePi[Sqrt[#]]-2] == FactorInteger[#][[1,1]] &] (* _Indranil Ghosh_, Mar 08 2017 *) %o A247394 (PARI) select(n->prime(primepi(sqrtint(n))-2)==factor(n)[1, 1], vector(10^4, x, x+24)) \\ _Jens Kruse Andersen_, Sep 17 2014 %Y A247394 Cf. A156759, A247393. %K A247394 nonn %O A247394 1,1 %A A247394 _Vladimir Shevelev_, Sep 16 2014 %E A247394 Terms up to a(54) from _Peter J. C. Moses_, Sep 16 2014