Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #20 Jul 04 2018 14:15:01
%S 14,15,21,22,26,27,33,34,35,38,39,57,58,85,86,87,93,94,95,118,119,122,
%T 123,133,134,141,142,143,145,146,158,159,177,178,201,202,203,205,206,
%U 213,214,215,217,218,219,253,254,298,299,301,302,303,326,327,334,335,381,382,393,394,395,445,446,447,453,454
%N List of pairs of consecutive numbers each with 4 divisors (duplicates removed).
%D D. Wells, Curious and interesting numbers, Penguin Books.
%H Harvey P. Dale, <a href="/A038456/b038456.txt">Table of n, a(n) for n = 1..1000</a>
%e 14 and 15 because both have 4 as number of divisors and are consecutive.
%t Union[Flatten[Select[Partition[Range[500],2,1],DivisorSigma[0,First[#]] == DivisorSigma[0,Last[#]]==4&]]] (* _Harvey P. Dale_, Jul 22 2012 *)
%t SequencePosition[DivisorSigma[0,Range[500]],{4,4}]//Flatten//Union (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 15 2016 *)
%o (PARI) isA038456(n) = (numdiv(n)==4) && ((numdiv(n+1)==4) || (numdiv(n-1)==4)) \\ _Michael B. Porter_, Feb 03 2010
%Y Cf. A039832, A039833.
%K nonn
%O 1,1
%A _Felice Russo_
%E Corrected and extended by _Olivier Gérard_
%E Corrected by _Rick L. Shepherd_, Jun 07 2002