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 A121494 Composite numbers k such that tau(k) = tau(2k+1). 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Composite terms of A074821, while prime terms of A074821 are Sophie Germain primes A005384. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 10 is a term since both 10 and 2*10+1=21 have 4 divisors: {1,2,5,10} and {1,3,7,21}. MATHEMATICA Select[Range[400], CompositeQ[#] && DivisorSigma[0, #] == DivisorSigma[0, 2*#+1] &] (* Amiram Eldar, Feb 18 2020 *) PROG (PARI) isok(n) = !isprime(n) && (numdiv(n) == numdiv(2*n+1)); \\ Michel Marcus, Oct 10 2013 (MAGMA) [k:k in [2..400]| not IsPrime(k) and #Divisors(k) eq #Divisors(2*k+1)]; // Marius A. Burtea, Feb 18 2020 CROSSREFS Cf. A000005, A005384, A074821. Sequence in context: A240048 A105999 A192210 * A122744 A192879 A077923 Adjacent sequences:  A121491 A121492 A121493 * A121495 A121496 A121497 KEYWORD nonn AUTHOR Zak Seidov, Aug 03 2006 STATUS approved

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Last modified January 22 23:50 EST 2022. Contains 350504 sequences. (Running on oeis4.)