cp's OEIS Frontend

A134865 Numbers k meeting the following criterion: if k is a multiple of d, then it is also a multiple of the smallest number with same number of divisors as d.

%I A134865 #24 Mar 17 2022 22:49:23
%S A134865 1,2,4,6,12,24,36,48,120,240,360,720,2520,5040,7560,10080,15120,20160,
%T A134865 45360,50400,100800,332640,352800,665280,705600,4324320,8648640,
%U A134865 17297280,21621600,43243200,13492656777600
%N A134865 Numbers k meeting the following criterion: if k is a multiple of d, then it is also a multiple of the smallest number with same number of divisors as d.
%C A134865 Note that this is not a subsequence of A002182: 100800 is in this sequence but not in A002182. - _J. Lowell_, Feb 22 2008
%C A134865 A subset of A005179. - _Max Alekseyev_, May 19 2008
%C A134865 A number k is in this sequence iff for every divisor d of k, A005179(A000005(d)) (= A140635(d)) is also a divisor of k. So the question of the finiteness of this sequence is closely related to the form of the elements of A005179. - _Max Alekseyev_, May 19 2008, May 20 2008
%C A134865 Rearrangement of this sequence, forming a subsequence of A005179, is given by A140753. Corresponding indices of elements of A005179 are given by A138394 and A140752. - _Max Alekseyev_, May 26 2008
%C A134865 A subsequence of A007416 which is a subsequence of A025487, so every term is primally tight and even (after the first term). Thus if d is a divisor of a term, then the least integer with the same prime signature as d (=A046523(d)) is also a divisor. So only the divisors that are in A025487 need be tested. - _Ray Chandler_
%C A134865 a(32) > 8*10^25 if it exists. - _David A. Corneth_, Dec 10 2021
%F A134865 a(n) = A005179(A140752(n)). - _Max Alekseyev_, May 26 2008
%e A134865 60 is a multiple of 30 with 8 divisors, but not of 24 (the smallest number with 8 divisors) so 60 is not a term of this sequence.
%t A134865 a = {}; For[n = 1, n < 10000, n++, b = Divisors[n]; c = 1; For[i = 1, i < Length[b] + 1, i++, j = 1; While[Length[Divisors[j]] < Length[Divisors[b[[i]]]], j++ ]; If[ ! Mod[n, j] == 0, c = 0]]; If[c == 1, AppendTo[a, n]]]; a (* _Stefan Steinerberger_, Feb 05 2008 *)
%o A134865 (PARI) isA134865(n)={ n%2 & return(n==1); fordiv(n, d, bigomega(d)>1 || next; nd=numdiv(d); for(k=4, d, numdiv(k)==nd || next; n%k & return; break)); 1 }
%o A134865 for(n=1,10^7,if(isA134865(n),print1(n,", "))); \\ _R. J. Mathar_, May 17 2008
%Y A134865 Cf. A000005, A005179, A007416, A138394, A140752, A140753.
%K A134865 more,nonn
%O A134865 1,2
%A A134865 _J. Lowell_, Jan 29 2008
%E A134865 More terms from _Stefan Steinerberger_, Feb 05 2008
%E A134865 More terms from _J. Lowell_, Feb 22 2008
%E A134865 a(22)-a(30) from _Don Reble_, May 17 2008
%E A134865 a(31)=13492656777600 from _Ray Chandler_, Jun 30 2008