cp's OEIS Frontend

A248202 Sphenic numbers (A007304) whose neighbors are sphenic.

%I A248202 #31 Dec 08 2024 16:53:03
%S A248202 1310,1886,2014,2666,3730,5134,6062,6214,6306,6478,6854,6986,7258,
%T A248202 7954,8394,8534,8786,9214,9454,9822,9878,10282,10946,11606,12454,
%U A248202 12566,12802,12858,12994,13054,14134,14314,14330,14466,14818,15086,15266,15806,16114,16134
%N A248202 Sphenic numbers (A007304) whose neighbors are sphenic.
%C A248202 Subsequence of A169834 and offset by 1 from the values in A066509.
%H A248202 Michael De Vlieger, <a href="/A248202/b248202.txt">Table of n, a(n) for n = 1..10000</a>
%H A248202 Wikipedia, <a href="http://en.wikipedia.org/wiki/Sphenic_number">Sphenic number</a>
%F A248202 a(n) = A066509(n)+1.
%e A248202 1309, 1310 and 1311 factor as 7*11*17, 2*5*131 and 3*19*23, respectively.  No smaller such trio exists, so a(1)=1310.
%t A248202 a248202[n_Integer] := Select[Range[n],
%t A248202   And[And[PrimeNu[#] == 3, PrimeNu[# - 1] == 3, PrimeNu[# + 1] == 3], And[PrimeOmega[#] == 3, PrimeOmega[# - 1] == 3, PrimeOmega[# + 1] == 3]] &]; a248202[20166](* _Michael De Vlieger_, Nov 06 2014 *)
%t A248202 f[n_]:=Last/@FactorInteger[n]=={1, 1, 1}; lst={}; Do[If[f[n]&&f[n+1]&&f[n+2], AppendTo[lst, n + 1]], {n, 17000}]; lst (* _Vincenzo Librandi_, Jul 24 2015 *)
%t A248202 Mean/@SequencePosition[Table[If[PrimeNu[n]==PrimeOmega[n]==3,1,0],{n,20000}],{1,1,1}] (* _Harvey P. Dale_, Dec 08 2024 *)
%o A248202 (PARI) sq(n)=bigomega(n)==3 && omega(n)==3;
%o A248202 for(n=3,10^5,if(sq(n-1)&&sq(n)&&sq(n+1),print1(n,", ")));
%o A248202 \\ _Joerg Arndt_, Oct 18 2014
%Y A248202 Cf. A007304, A169834, A066509, A248201, A248203, A248204, A259349, A259350, A259801.
%K A248202 nonn
%O A248202 1,1
%A A248202 _James G. Merickel_, Oct 03 2014