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 A247393 #38 Feb 26 2025 05:54:03 %S A247393 10,12,14,16,18,20,22,24,27,33,39,45,55,65,85,95,115,133,161,187,209, %T A247393 253,299,391,493,527,551,589,703,779,817,851,943,1073,1189,1247,1363, %U A247393 1457,1643,1739,1927,2173,2279,2537,2623,2867,3149,3337,3431,3551,3953 %N A247393 Numbers n such that the second maximal prime <= sqrt(n) is the least prime divisor of n. %C A247393 These numbers we call "preprimes" of the second kind in contrast to A156759 for n>=2, for which the maximal prime <= sqrt(n) is the least prime divisor of n. Terms of A156759 (n>=2) we call "preprimes" (cf. comment there). %H A247393 Jens Kruse Andersen, <a href="/A247393/b247393.txt">Table of n, a(n) for n = 1..10000</a> %H A247393 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 A247393 lpf(a(n)) = prime(pi(sqrt(a(n)))-1), where pi(n) = A000720(n). %e A247393 a(1)=10. Indeed, in interval [2,sqrt(10)] we have two primes: 2 and 3. Maximal from them 3, the second maximal is 2, and 2=lpf(10). %t A247393 Select[Range[4000], Prime[PrimePi[Sqrt[#]]-1] == FactorInteger[#][[1,1]] &] (* _Indranil Ghosh_, Mar 08 2017 *) %o A247393 (PARI) select(n->prime(primepi(sqrtint(n))-1)==factor(n)[1, 1], vector(10^4, x, x+8)) \\ _Jens Kruse Andersen_, Sep 17 2014 %Y A247393 Cf. A156759. %K A247393 nonn %O A247393 1,1 %A A247393 _Vladimir Shevelev_, Sep 16 2014 %E A247393 More terms from _Peter J. C. Moses_, Sep 16 2014 %E A247393 a(52..10000) from _Jens Kruse Andersen_, Sep 17 2014