A077646 Solutions to lcm(n, tau(n)) = 2n which are neither prime nor twice a prime, where tau(k) = d(k) is the number of divisors of k.

45, 63, 75, 90, 99, 117, 120, 126, 147, 150, 153, 168, 171, 198, 207, 216, 234, 243, 261, 264, 279, 280, 294, 306, 312, 333, 342, 363, 369, 387, 405, 408, 414, 420, 423, 440, 456, 477, 486, 507, 520, 522, 531, 540, 549, 552, 558, 603, 616, 639, 657, 660

Offset

1,1

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Formula

Composite solutions to A009530(x)=2x which are not twice a prime.

Example

n=63: d(n) = tau(n) = 6, lcm(63,6) = 126 = 2n.

Prog

(PARI) is(n)=!isprime(if(n%2, n, n/2)) && lcm(n, numdiv(n))==2*n \\ Charles R Greathouse IV, Oct 28 2013

Crossrefs

Cf. A009530, A001747, A000040, A065091.

Sequence in context: A242263 A242264 A333326 * A230577 A336555 A175761

Adjacent sequences: A077643 A077644 A077645 * A077647 A077648 A077649

Keyword

nonn

Author

Labos Elemer, Nov 18 2002

Status

approved