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 A237040 #24 Feb 16 2025 08:33:21 %S A237040 9,65,217,4097,5833,10649,21953,74089,195113,216001,343001,373249, %T A237040 474553,1000001,1061209,1191017,1404929,3241793,3796417,4251529, %U A237040 6859001,9261001,12487169,21952001,29791001,35937001,43614209,45882713,55742969,62099137,89915393,94818817,117649001 %N A237040 Semiprimes of the form k^3 + 1. %C A237040 k^3 + 1 is a term iff k + 1 and k^2 - k + 1 are both prime. %C A237040 Is the sequence infinite? This is an analog of Landau's 4th problem, namely, are there infinitely many primes of the form k^2 + 1? %C A237040 In other words: are there infinitely many primes p such that p^2 - 3*p + 3 is also prime? - _Charles R Greathouse IV_, Jul 02 2017 %H A237040 Vincenzo Librandi, <a href="/A237040/b237040.txt">Table of n, a(n) for n = 1..1400</a> %H A237040 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a> %H A237040 Wikipedia, <a href="https://en.wikipedia.org/wiki/Semiprime">Semiprime</a> %H A237040 Wikipedia, <a href="https://en.wikipedia.org/wiki/Landau's_problems">Landau's problems</a> %F A237040 a(n) = A096173(n)^3 + 1 = 8*A237037(n)^3 + 1. %e A237040 9 = 3*3 = 2^3 + 1 is the first semiprime of the form n^3 + 1, so a(1) = 9. %t A237040 L = Select[Range[500], PrimeQ[# + 1] && PrimeQ[#^2 - # + 1] &]; L^3 + 1 %t A237040 Select[Range[50]^3 + 1, PrimeOmega[#] == 2 &] (* _Zak Seidov_, Jun 26 2017 *) %o A237040 (PARI) lista(nn) = for (n=1, nn, if (bigomega(sp=n^3+1) == 2, print1(sp, ", "));); \\ _Michel Marcus_, Jun 27 2017 %o A237040 (PARI) list(lim)=my(v=List(),n,t); forprime(p=3,sqrtnint(lim\1-1,3)+1, if(isprime(t=p^2-3*p+3), listput(v,t*p))); Vec(v) \\ _Charles R Greathouse IV_, Jul 02 2017 %o A237040 (Magma) IsSemiprime:= func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [1..500] | IsSemiprime(s) where s is n^3 + 1]; // _Vincenzo Librandi_, Jul 02 2017 %Y A237040 Cf. A001358, A002383, A002496, A046315, A081256, A096173, A096174, A237037, A237038, A237039. %Y A237040 Cf. A242262 (semiprimes of the form k^3 - 1). %K A237040 nonn %O A237040 1,1 %A A237040 _Jonathan Sondow_, Feb 02 2014