This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A121494 #14 Sep 08 2022 08:45:27
%S A121494 4,10,27,34,38,46,55,57,62,76,77,91,93,106,118,123,129,133,136,143,
%T A121494 145,159,161,177,185,201,203,205,206,212,213,218,226,232,235,259,267,
%U A121494 291,295,297,298,305,310,314,322,327,334,335,339,343,357,358,365,370,377
%N A121494 Composite numbers k such that tau(k) = tau(2k+1).
%C A121494 Composite terms of A074821, while prime terms of A074821 are Sophie Germain primes A005384.
%H A121494 Amiram Eldar, <a href="/A121494/b121494.txt">Table of n, a(n) for n = 1..10000</a>
%e A121494 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 A121494 Select[Range[400], CompositeQ[#] && DivisorSigma[0, #] == DivisorSigma[0, 2*#+1] &] (* _Amiram Eldar_, Feb 18 2020 *)
%o A121494 (PARI) isok(n) = !isprime(n) && (numdiv(n) == numdiv(2*n+1)); \\ _Michel Marcus_, Oct 10 2013
%o A121494 (Magma) [k:k in [2..400]| not IsPrime(k) and #Divisors(k) eq #Divisors(2*k+1)]; // _Marius A. Burtea_, Feb 18 2020
%Y A121494 Cf. A000005, A005384, A074821.
%K A121494 nonn
%O A121494 1,1
%A A121494 _Zak Seidov_, Aug 03 2006