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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

Labos Elemer

STATUS

approved

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