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A020522 a(n) = 4^n - 2^n. 28
0, 2, 12, 56, 240, 992, 4032, 16256, 65280, 261632, 1047552, 4192256, 16773120, 67100672, 268419072, 1073709056, 4294901760, 17179738112, 68719214592, 274877382656, 1099510579200, 4398044413952, 17592181850112, 70368735789056, 281474959933440 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Number of walks of length 2n+2 between any two diametrically opposite vertices of the cycle graph C_8. - Herbert Kociemba, Jul 02 2004

If we consider a(4k+2), then 2^4 == 3^4 == 3 (mod 13); 2^(4k+2) + 3^(4k+2) == 3^k(4+9) == 3*0 == 0 (mod 13). So a(4k+2) can never be prime. - Jose Brox, Dec 27 2005

If k is odd, then a(nk) is divisible by a(n), since: a(nk) = (2^n)^k + (3^n)^k = (2^n + 3^n) [(2^n)^(k-1) - (2^n)^(k-2) (3^n) + - ... + (3^n)^(k-1)]. So the only possible primes in the sequence are a(0) and a(2^n) for n>=1. I've checked that a(2^n) is composite for 3 <= n <= 15. As with Fermat primes, a probabilistic argument suggests that there are only finitely many primes in the sequence. - Dean Hickerson, Dec 27 2005

Let x,y,z be elements from some power set P(n), i.e. the power set of a set of n elements. Define a function f(x,y,z) in the following manner: f(x,y,z) = 1 if x is a subset of y and y is a subset of z and x does not equal z; f(x,y,z) = 0 if x is not a subset of y or y is not a subset of z or x equals z. Now sum f(x,y,z) for all x,y,z of P(n). This gives a(n). - Ross La Haye, Dec 26 2005

Number of monic (irreducible) polynomials of degree 1 over GF(2^n). - Max Alekseyev, Jan 13 2006

a(n) = A099393(n)-A000225(n+1) = A083420(n)-A099393(n); in binary representation, n>0: n ones followed by n zeros (A138147(n)); A000120(a(n))=n; A023416(a(n))=n; A070939(a(n))=2*n; 2*a(n)+1=A030101(A099393(n)). - Reinhard Zumkeller, Feb 07 2006, Jaroslav Krizek, Aug 02 2009

Let P(A) be the power set of an n-element set A and B be the Cartesian product of P(A) with itself. Then a(n) = the number of (x,y) of B for which x does not equal y. - Ross La Haye, Jan 02 2008

For n>1: central terms of the triangle in A173787. - Reinhard Zumkeller, Feb 28 2010

Pronic numbers of the form: (2^n - 1) * 2^n, which is the n-th Mersenne number times 2^n, see A000225 and A002378. - Fred Daniel Kline, Nov 30 2013

Indices where records of A037870 occur. - Philippe Beaudoin, Sep 03 2014

a(n) = 2*A006516(n) = A000079(n)*A000225(n) = A265736(A000225(n)). - Reinhard Zumkeller, Dec 15 2015

Total edge length for a minimum linear arrangement of a hypercube of dimension n. (See Harper's paper below for proof). - Eitan Frachtenberg, Apr 07 2017

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..170

T. Copeland, The Kervaire-Milnor formula

Ross La Haye, Binary Relations on the Power Set of an n-Element Set, Journal of Integer Sequences, Vol. 12 (2009), Article 09.2.6.

Question A6, The Sixtieth William Lowell Putnam Mathematical Competition, Amer. Math. Monthly 107 (Oct 2000), 721-732; see p. 725.

Index entries for linear recurrences with constant coefficients, signature (6,-8).

L. H. Harper, Optimal Assignment of Numbers to Vertices, J. SIAM 12(1), p. 131--135, March 1964.

FORMULA

G.f.: 2x/((-1 + 2x)(-1 + 4x)), a(n)=6a(n-1)-8a(n-2). - Herbert Kociemba, Jul 02 2004

E.g.f.: e^(4*x)-e^(2*x). - Mohammad K. Azarian, Jan 14 2009

a(n) = A085812(n) - A001700(n). - John Molokach, Sep 28 2013

EXAMPLE

n=5: a(5)=4^5-2^5=1024-32=992 -> '1111100000'.

MAPLE

A020522:=n->4^n-2^n; seq(A020522(n), n=0..50); # Wesley Ivan Hurt, Nov 29 2013

MATHEMATICA

Table[4^n - 2^n, {n, 40}] (* or *) LinearRecurrence[{6, -8}, {0, 2}, 40] (* Vladimir Joseph Stephan Orlovsky, Feb 20 2012 *)

PROG

(Sage) [4^n - 2^n for n in xrange(0, 23)] # Zerinvary Lajos, Jun 05 2009

(MAGMA) [4^n - 2^n: n in [0..60]]; // Vincenzo Librandi, Apr 26 2011

(PARI) a(n)=4^n-2^n \\ Charles R Greathouse IV, Jan 30 2012

(Haskell)

a020522 = (* 2) . a006516  -- Reinhard Zumkeller, Dec 15 2015

CROSSREFS

Ratio of successive terms of A028365.

Cf. A000225, A060867, A161168, A006516, A059153.

Cf. A000079, A265736.

Sequence in context: A006659 A194771 A127221 * A037130 A181298 A247121

Adjacent sequences:  A020519 A020520 A020521 * A020523 A020524 A020525

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane, Simon Plouffe

STATUS

approved

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Last modified June 22 12:24 EDT 2017. Contains 288613 sequences.