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 A036537 Numbers whose number of divisors is a power of 2. 15
 1, 2, 3, 5, 6, 7, 8, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 24, 26, 27, 29, 30, 31, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 46, 47, 51, 53, 54, 55, 56, 57, 58, 59, 61, 62, 65, 66, 67, 69, 70, 71, 73, 74, 77, 78, 79, 82, 83, 85, 86, 87, 88, 89, 91, 93, 94, 95, 97, 101, 102 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Primes and A030513(d(x)=4) are subsets; d(16k+4) and d(16k+12) have the form 3Q, so x=16k+4 or 16k-4 numbers are missing. A number m is a term if and only if all its divisors are infinitary, or A000005(m) = A037445(m). - Vladimir Shevelev, Feb 23 2017 All exponents in the prime number factorization of a(n) have the form 2^k-1, k >= 1. So it is an S-exponential sequence (see Shevelev link) with S={2^k-1}. Using Theorem 1, we obtain that a(n) ~ C*n, where C = Product((1-1/p)*(1 + Sum_{i>=1} 1/p^(2^i-1))). - Vladimir Shevelev Feb 27 2017 This constant is C = 0.687827... . - Peter J. C. Moses, Feb 27 2017 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Vladimir Shevelev, S-exponential numbers, Acta Arithm., 175(2016), 385-395. FORMULA A209229(A000005(a(n))) = 1. - Reinhard Zumkeller, Nov 15 2012 a(n) << n. - Charles R Greathouse IV, Feb 25 2017 EXAMPLE 383, 384, 385, 386 have 1, 16, 8, 4 divisors, respectively, so they are consecutive terms of this sequence. MATHEMATICA bi[ x_ ] := 1-Sign[ N[ Log[ 2, x ], 5 ]-Floor[ N[ Log[ 2, x ], 5 ] ] ]; ld[ x_ ] := Length[ Divisors[ x ] ]; Flatten[ Position[ Table[ bi[ ld[ x ] ], {x, 1, m} ], 1 ] ] Select[Range[110], IntegerQ[Log[2, DivisorSigma[0, #]]]&] (* Harvey P. Dale, Nov 20 2016 *) PROG (Haskell) a036537 n = a036537_list !! (n-1) a036537_list = filter ((== 1) . a209229 . a000005) [1..] -- Reinhard Zumkeller, Nov 15 2012 (PARI) is(n)=n=numdiv(n); n>>valuation(n, 2)==1 \\ Charles R Greathouse IV, Mar 27 2013 CROSSREFS A005117 is a subsequence. Complement of A162643; subsequence of A002035. - Reinhard Zumkeller, Jul 08 2009 Cf. A000005, A030513, A036538, A162644. Sequence in context: A162644 A268335 A002035 * A072510 A084116 A137620 Adjacent sequences:  A036534 A036535 A036536 * A036538 A036539 A036540 KEYWORD nonn AUTHOR STATUS approved

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Last modified September 18 11:42 EDT 2018. Contains 315130 sequences. (Running on oeis4.)