A308643 Odd squarefree composite numbers k, divisible by the sum of their prime factors, sopfr (A001414).

105, 231, 627, 805, 897, 1581, 2967, 3055, 4543, 5487, 6461, 6745, 7881, 9717, 10707, 14231, 15015, 16377, 21091, 26331, 29607, 33495, 33901, 33915, 35905, 37411, 38843, 40587, 42211, 45885, 49335, 50505, 51051, 53295, 55581, 60297

Offset

1,1

Comments

Every term has an odd number of prime divisors (A001221(k) is odd), since if not, sopfr(k) would be even, and so not divide k, which is odd.

Some Carmichael numbers appear in this sequence, the first of which is 3240392401.

From Robert Israel, Jul 05 2019: (Start)

Includes p*q*r if p and q are distinct odd primes and r=(p-1)*(q-1)-1 is prime. Dickson's conjecture implies that there are infinitely many such terms for each odd prime p. Thus for p=3, q is in A063908 (except 3), for p=5, q is in A156300 (except 2), and for p=7, q is in A153135 (except 2). (End)

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

105=3*5*7; sum of prime factors = 15 and 105 = 7*15, so 105 is a term.

Maple

with(NumberTheory);
N := 500;
for n from 2 to N do
S := PrimeFactors(n);
X := add(S);
if IsSquareFree(n) and not mod(n, 2) = 0 and not isprime(n) and mod(n, X) = 0 then print(n);
end if:
end do:

Mathematica

aQ[n_] := Module[{f = FactorInteger[n]}, p=f[[;; , 1]]; e=f[[;; , 2]]; Length[e] > 1 && Max[e]==1 && Divisible[n, Plus@@(p^e)]]; Select[Range[1, 61000, 2], aQ] (* Amiram Eldar, Jul 04 2019 *)

Prog

(Magma) [k:k in [2*d+1: d in [1..35000]]|IsSquarefree(k) and not IsPrime(k) and k mod &+PrimeDivisors(k) eq 0]; // Marius A. Burtea, Jun 19 2019

Crossrefs

Intersection of A005117 and A046347.

Cf. A001414, A046346, A002997, A001221, A063908, A156300, A153135.

Sequence in context: A179143 A176878 A088595 * A229094 A307108 A262723

Adjacent sequences: A308640 A308641 A308642 * A308644 A308645 A308646

Keyword

nonn

Author

David James Sycamore, Jun 13 2019

Status

approved