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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. 2

%I

%S 45,63,75,90,99,117,120,126,147,150,153,168,171,198,207,216,234,243,

%T 261,264,279,280,294,306,312,333,342,363,369,387,405,408,414,420,423,

%U 440,456,477,486,507,520,522,531,540,549,552,558,603,616,639,657,660

%N 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.

%H Charles R Greathouse IV, <a href="/A077646/b077646.txt">Table of n, a(n) for n = 1..10000</a>

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

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

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

%Y Cf. A009530, A001747, A000040, A065091.

%K nonn

%O 1,1

%A _Labos Elemer_, Nov 18 2002

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Last modified October 14 12:31 EDT 2019. Contains 328006 sequences. (Running on oeis4.)