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A036537 Numbers whose number of divisors is a power of 2. 31
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

From Peter Munn, Jun 18 2022: (Start)

1 and numbers j*m^2, j squarefree, m >= 1, such that all prime divisors of m divide j, and m is in the sequence.

Equivalently, the nonempty set of numbers whose squarefree part (A007913) and squarefree kernel (A007947) are equal, and whose square part's square root (A000188) is in the set.

(End)

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Vladimir Shevelev, S-exponential numbers, Acta Arithm., 175(2016), 385-395.

Eric Weisstein's World of Mathematics, Square Part, Squarefree Part

FORMULA

A209229(A000005(a(n))) = 1. - Reinhard Zumkeller, Nov 15 2012

a(n) << n. - Charles R Greathouse IV, Feb 25 2017

m is in the sequence iff for k >= 0, A352780(m, k+1) | A352780(m, k)^2. - Peter Munn, Jun 18 2022

EXAMPLE

383, 384, 385, 386 have 2, 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

(PARI) isok(m) = issquarefree(m) || (omega(m) == omega(core(m)) && isok(core(m, 1)[2])); \\ Peter Munn, Jun 18 2022

CROSSREFS

A005117, A030513, A058891, A175496, A336591 are subsequences.

Complement of A162643; subsequence of A002035. - Reinhard Zumkeller, Jul 08 2009

Subsequence of A162644, A337533.

Cf. A000005, A000188, A007913, A007947, A036538, A352780.

The closure of the squarefree numbers under application of A355038(.) and lcm.

Sequence in context: A268335 A002035 A336591 * A072510 A084116 A137620

Adjacent sequences:  A036534 A036535 A036536 * A036538 A036539 A036540

KEYWORD

nonn

AUTHOR

Labos Elemer

STATUS

approved

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Last modified August 8 10:26 EDT 2022. Contains 356009 sequences. (Running on oeis4.)