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%I #14 Sep 08 2022 08:45:27
%S 4,10,27,34,38,46,55,57,62,76,77,91,93,106,118,123,129,133,136,143,
%T 145,159,161,177,185,201,203,205,206,212,213,218,226,232,235,259,267,
%U 291,295,297,298,305,310,314,322,327,334,335,339,343,357,358,365,370,377
%N Composite numbers k such that tau(k) = tau(2k+1).
%C Composite terms of A074821, while prime terms of A074821 are Sophie Germain primes A005384.
%H Amiram Eldar, <a href="/A121494/b121494.txt">Table of n, a(n) for n = 1..10000</a>
%e 10 is a term since both 10 and 2*10+1=21 have 4 divisors: {1,2,5,10} and {1,3,7,21}.
%t Select[Range[400], CompositeQ[#] && DivisorSigma[0, #] == DivisorSigma[0, 2*#+1] &] (* _Amiram Eldar_, Feb 18 2020 *)
%o (PARI) isok(n) = !isprime(n) && (numdiv(n) == numdiv(2*n+1)); \\ _Michel Marcus_, Oct 10 2013
%o (Magma) [k:k in [2..400]| not IsPrime(k) and #Divisors(k) eq #Divisors(2*k+1)]; // _Marius A. Burtea_, Feb 18 2020
%Y Cf. A000005, A005384, A074821.
%K nonn
%O 1,1
%A _Zak Seidov_, Aug 03 2006
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