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A151821 Powers of 2, omitting 2 itself. 24
1, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648, 4294967296, 8589934592 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Different from A046055.

An elephant sequence, see A175655. For the central square just one A[5] vector, with decimal value 170, leads to this sequence. For the corner squares this vector leads to the companion sequence A095121. - Johannes W. Meijer, Aug 15 2010

This is a subsequence of A055744, numbers n such that n and phi(n) have same prime factors. - Michel Marcus, Mar 20 2015

INVERTi transform of A007483: (1, 5, 17, 61, 217, 773, ...). - Gary W. Adamson, Aug 06 2016

Nonprimes that are also powers of 2. Intersection of A000079 and A018252. - Omar E. Pol, Jan 27 2017

Also the chromatic number of the n-Keller graph. - Eric W. Weisstein, Nov 17 2017

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Eric Weisstein's World of Mathematics, Chromatic Number

Eric Weisstein's World of Mathematics, Keller Graph

FORMULA

G.f.: x*(1+2*x)/(1-2*x). - Philippe Deléham, Sep 17 2009

a(1) = 1 and a(n) = 3 + Sum_{k=1..n-1} a(k) for n>=2. - Joerg Arndt, Aug 15 2012

MATHEMATICA

CoefficientList[Series[(1 + 2 x)/(1 - 2 x), {x, 0, 40}], x] (* Vincenzo Librandi, Jul 21 2013

DeleteCases[2^Range[0, 33], p_ /; PrimeQ @ p] (* Michael De Vlieger, Aug 06 2016 *)

Join[{1}, 2^Range[2, 20]] (* Eric W. Weisstein, Nov 17 2017 *)

PROG

(MAGMA) [1] cat [2^n: n in [2..35]]; // Vincenzo Librandi, Jul 21 2013

(Haskell)

a151821 n = a151821_list !! (n-1)

a151821_list = x : xs where (x : _ : xs) = a000079_list

-- Reinhard Zumkeller, Dec 16 2013

(PARI) a(n)=if(n>1, 2^n, 1) \\ Charles R Greathouse IV, Dec 08 2015

(PARI) Vec(x*(1+2*x)/(1-2*x) + O(x^100)) \\ Altug Alkan, Dec 09 2015

CROSSREFS

Cf. A000079, A000225, A007483, A063759.

Partial sums are given by 2*A000225(n)-1, which is not the same as A000918.

Sequence in context: A046055 A186949 A020707 * A147639 A049934 A089890

Adjacent sequences:  A151818 A151819 A151820 * A151822 A151823 A151824

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Jul 08 2009

STATUS

approved

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Last modified February 23 23:15 EST 2018. Contains 299595 sequences. (Running on oeis4.)