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 A115170 #20 May 25 2025 01:58:50 %S A115170 40,256,312,472,688,904,1120,1200,1312,1336,1552,1768,1984,2187,2200, %T A115170 2312,2416,2456,2632,2848,2875,3064,3280,3312,3429,3496,3712,3928, %U A115170 3944,4144,4312,4360,4576,4792,5008,5224,5312,5440,5562,5656,5872,6088,6250,6304,6312,6345,6520,6655,6688,6736,6952 %N A115170 Non-cubefree numbers k such that 2k+1 is also non-cubefree (A046099). %C A115170 The probability that a random integer is cubefree is 1/zeta(3) (see A088453). %H A115170 Amiram Eldar, <a href="/A115170/b115170.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..700 from R. J. Mathar) %e A115170 312 is there because 2^3 divides 312 and 5^3 divides 312*2+1. %t A115170 noncfQ[n_] := AnyTrue[FactorInteger[n][[;; , 2]], # > 2 &]; Select[Range[7000], noncfQ[#] && noncfQ[2*# + 1] &] (* _Amiram Eldar_, May 25 2025 *) %Y A115170 Cf. A005384 (n and 2n+1 prime), A046099, A088453, A111153 (n and 2n+1 semiprime), A117204 (n and 2n+1 squarefree), A115228. %K A115170 easy,nonn %O A115170 1,1 %A A115170 _Jonathan Vos Post_, Mar 03 2006 %E A115170 Edited by _Don Reble_, Mar 05 2006 %E A115170 2875 inserted by _R. J. Mathar_, Dec 08 2015