A121494 Composite numbers k such that tau(k) = tau(2k+1).

4, 10, 27, 34, 38, 46, 55, 57, 62, 76, 77, 91, 93, 106, 118, 123, 129, 133, 136, 143, 145, 159, 161, 177, 185, 201, 203, 205, 206, 212, 213, 218, 226, 232, 235, 259, 267, 291, 295, 297, 298, 305, 310, 314, 322, 327, 334, 335, 339, 343, 357, 358, 365, 370, 377

Offset: 1

Author

Zak Seidov, Aug 03 2006

Keywords

• nonn

Comments

Composite terms of A074821, while prime terms of A074821 are Sophie Germain primes A005384.

Examples

			10 is a term since both 10 and 2*10+1=21 have 4 divisors: {1,2,5,10} and {1,3,7,21}.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000005, A005384, A074821.

Programs

Magma

[k:k in [2..400]| not IsPrime(k) and #Divisors(k) eq #Divisors(2*k+1)]; // Marius A. Burtea, Feb 18 2020

Mathematica

Select[Range[400], CompositeQ[#] && DivisorSigma[0, #] == DivisorSigma[0, 2*#+1] &] (* Amiram Eldar, Feb 18 2020 *)

PARI

isok(n) = !isprime(n) && (numdiv(n) == numdiv(2*n+1)); \\ Michel Marcus, Oct 10 2013