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 A227006 #21 Jul 30 2025 19:31:09
%S A227006 5,6,9,10,11,12,13,14,16,17,18,19,20,21,23,24,25,26,27,28,29,30,31,32,
%T A227006 33,34,35,36,37,38,39,40,41,42,45,46,47,48,49,50,51,52,53,54,55,56,57,
%U A227006 58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75
%N A227006 Numbers k such that k-1 is not squarefree or a prime divisor of k-1 is in the sequence.
%C A227006 The sequence is defined recursively. The complement of A227007.
%H A227006 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A227006 a(n) = n + 16 for n > 1791. - _Charles R Greathouse IV_, Jun 27 2013
%t A227006 Needs["NumberTheory`NumberTheoryFunctions`"];Property[2] = False; Property[{}] = False; Property[n_] := Property[n] = If[ListQ[n],Property[n[[1, 1]]] || Property[Rest[n]], SquareFreeQ[n - 1] == False || Property[fa[n - 1]]]; Select[1 + Range[100], Property]
%o A227006 (PARI) is(n)=if(n<7,return(n>4)); if(n>1807 || !issquarefree(n-1), return(1)); fordiv(n-1,d,if(isprime(d) && is(d), return(1))); 0 \\ _Charles R Greathouse IV_, Nov 13 2013
%Y A227006 Cf. A227007.
%K A227006 nonn,easy
%O A227006 1,1
%A A227006 _José María Grau Ribas_, Jun 27 2013
%E A227006 Definition corrected by _Charles R Greathouse IV_, Nov 13 2013