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).

15, 77, 1403, 3127, 3139, 8383, 15247, 45151, 47263, 54053, 58339, 65473, 73813, 79567, 89951, 94957, 155011, 211621, 293323, 333961, 360883, 441901, 444853, 496597, 612893, 623659, 646367, 727393, 786193, 796723, 1334083, 1456813, 1572491, 2103379, 2139793, 2477509, 2668867, 2735539, 2826787, 2903591, 3121133

Offset: 1

Author

Antti Karttunen, Jul 07 2024

Keywords

• nonn

Comments

The first 47 terms are all semiprimes.

For three consecutive terms k=293323, 333961, 360883, A347381(k) = 89.

For three consecutive terms k=612893, 623659, 646367, A347381(k) = 134.

Examples

			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.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..47
• Index entries for sequences related to sigma(n)

Crossrefs

Cf. A000203, A005940, A347381.

Subsequence of A374218.

Programs

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));