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 A261460 #13 Sep 08 2022 08:46:13 %S A261460 2,20,30,60,212,224,258,272,390,398,480,504,654,770,812,1040,1194, %T A261460 1448,1698,1748,1874,2000,2238,2274,2294,2438,2522,2664,2714,2790, %U A261460 2802,3020,3138,3168,3300,3392,3434,3794,4160,4232,4518,4722,4968,5334,5654,5658 %N A261460 Numbers k such that k^11-1 is a semiprime. %C A261460 After 2, numbers k such that k-1 and k^10 + k^9 + k^8 + k^7 + k^6 + k^5 + k^4 + k^3 + k^2 + k + 1 are both prime. %C A261460 Intersection of A008864 and A162862. - _Michel Marcus_, Aug 21 2015 %e A261460 20 is in sequence because 20^11-1 = 204799999999999 = 19*10778947368421, where 19 and 10778947368421 are both prime. %t A261460 Select[Range[6000], PrimeOmega[#^11 - 1] == 2 &] %o A261460 (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..4000] | IsSemiprime(s) where s is n^11- 1]; %o A261460 (PARI) isok(n)=bigomega(n^11-1)==2 \\ _Anders Hellström_, Aug 21 2015 %Y A261460 Cf. similar sequences listed in A261435. %Y A261460 Cf. A105122. %Y A261460 Cf. A008864, A162862. %K A261460 nonn,easy %O A261460 1,1 %A A261460 _Vincenzo Librandi_, Aug 21 2015