A230605 - OEIS

%I #21 Feb 23 2014 07:22:03

%S 4,18,20,24,432,588,692945344

%N Composite numbers n such that number of non-divisors of n divides sum of non-divisors of n.

%C a(7) > 10^5.

%C Composite numbers n such that A231167(n) mod A049820(n) = 0. Complement of primes (A000040) with respect to A140826. Intersection of A002808 and A140826.

%C The terms 20 and 432 are the only composite numbers < 10^5 that are also terms of A023883.

%C No more terms up to 10^8. - _Michel Marcus_, Nov 09 2013

%C a(8) > 10^11. - _Donovan Johnson_, Nov 12 2013

%C a(8) > 4*10^12. - _Giovanni Resta_, Feb 11 2014

%e 20 is in sequence because A024816(20) mod A049820(20) = 168 mod 14 = 0.

%o (PARI) isok(n) = (nnd = n - numdiv(n)) && !isprime(n) && !((n*(n+1)/2-sigma(n)) % nnd); \\ _Michel Marcus_, Nov 09 2013

%Y Cf. A231167, A000040, A049820, A024816.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Nov 07 2013

%E a(7) from _Donovan Johnson_, Nov 10 2013