cp's OEIS Frontend

A374219 Composite numbers k such that A347381 obtains the same value for all divisors of k that are larger than one, where A347381 is the distance from n to the nearest common ancestor of n and sigma(n) in the Doudna-tree (A005940).

%I A374219 #13 Jul 12 2024 21:16:36
%S A374219 15,77,1403,3127,3139,8383,15247,45151,47263,54053,58339,65473,73813,
%T A374219 79567,89951,94957,155011,211621,293323,333961,360883,441901,444853,
%U A374219 496597,612893,623659,646367,727393,786193,796723,1334083,1456813,1572491,2103379,2139793,2477509,2668867,2735539,2826787,2903591,3121133
%N A374219 Composite numbers k such that A347381 obtains the same value for all divisors of k that are larger than one, where A347381 is the distance from n to the nearest common ancestor of n and sigma(n) in the Doudna-tree (A005940).
%C A374219 The first 47 terms are all semiprimes.
%C A374219 For three consecutive terms k=293323, 333961, 360883, A347381(k) = 89.
%C A374219 For three consecutive terms k=612893, 623659, 646367, A347381(k) = 134.
%H A374219 Antti Karttunen, <a href="/A374219/b374219.txt">Table of n, a(n) for n = 1..47</a>
%H A374219 <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>
%e A374219 77 has divisors [7, 11, 77] that are larger than 1. For all of them, A347381 obtains value 3, therefore 77 is included in the sequence.
%o A374219 (PARI) isA374219(n) = if(1==n || isprime(n), 0, my(w=A347381(n)); fordiv(n, d, if(d>1 && A347381(d)!=w, return(0))); (1));
%Y A374219 Cf. A000203, A005940, A347381.
%Y A374219 Subsequence of A374218.
%K A374219 nonn
%O A374219 1,1
%A A374219 _Antti Karttunen_, Jul 07 2024