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

Author

Labos Elemer, Nov 18 2002

Keywords

• nonn

Examples

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

Links

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

Crossrefs

Cf. A009530, A001747, A000040, A065091.

Programs

PARI

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

Formula

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