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A007583 a(n) = (2^(2*n+1) + 1)/3.
(Formerly M2895)
62

%I M2895

%S 1,3,11,43,171,683,2731,10923,43691,174763,699051,2796203,11184811,

%T 44739243,178956971,715827883,2863311531,11453246123,45812984491,

%U 183251937963,733007751851,2932031007403,11728124029611,46912496118443

%N a(n) = (2^(2*n+1) + 1)/3.

%C Let u(k), v(k), w(k) be the 3 sequences defined by u(1)=1, v(1)=0, w(1)=0 and u(k+1)=u(k)+v(k)-w(k), v(k+1)=u(k)-v(k)+w(k), w(k+1)=-u(k)+v(k)+w(k); let M(k)=Max(u(k),v(k),w(k)); then a(n)=M(2n)=M(2n-1). - _Benoit Cloitre_, Mar 25 2002

%C Also the number of words of length 2n generated by the two letters s and t that reduce to the identity 1 by using the relations ssssss=1, tt=1 and stst=1. The generators s and t along with the three relations generate the dihedral group D6=C2xD3. - Jamaine Paddyfoot (jay_paddyfoot(AT)hotmail.com) and _John W. Layman_, Jul 08 2002

%C Binomial transform of A025192. - _Paul Barry_, Apr 11 2003

%C Number of walks of length 2n+1 between two adjacent vertices in the cycle graph C_6. Example: a(1)=3 because in the cycle ABCDEF we have three walks of length 3 between A and B: ABAB, ABCB and AFAB. - _Emeric Deutsch_, Apr 01 2004

%C Numbers of the form 1 + Sum_{i=1..m} [2^(2i-1)]. - _Artur Jasinski_, Feb 09 2007

%C Prime numbers of the form 1+Sum[2^(2n-1)] are in A000979. Numbers x such 1+Sum[2^(2n-1)] is prime for n=1,2,...,x is A127936. - _Artur Jasinski_, Feb 09 2007

%C Related to A024493(6n+1), A131708(6n+3), A024495(6n+5). - _Paul Curtz_, Mar 27 2008

%C Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=-6, (i>1), A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=1, a(n-1)=(-1)^(n-1)*charpoly(A,2). - _Milan Janjic_, Feb 21 2010

%C Number of toothpicks in the toothpick structure of A139250 after 2^n stages. - _Omar E. Pol_, Feb 28 2011

%C Numbers whose binary representation is 10, n-1 times, together with 11, n >= 1. For example 171 = 10101011 (2). - _Omar E. Pol_, Nov 22 2012

%C a(n) is the smallest number for which A072219(a(n)) = 2*n+1. - _Ramasamy Chandramouli_, Dec 22 2012.

%C An Engel expansion of 2 to the base b := 4/3 as defined in A181565, with the associated series expansion 2 = b + b^2/3 + b^3/(3*11) + b^4/(3*11*43) + .... Cf. A007051. - _Peter Bala_, Oct 29 2013

%C The positive integer solution (x,y) of 3*x - 2^n*y = 1, n>=0, with smallest x is (a(n/2), 2) if n is even and (a((n-1)/2), 1) if n is odd. - _Wolfdieter Lang_, Feb 15 2014

%D H. W. Gould, Combinatorial Identities, Morgantown, 1972, (1.77), page 10.

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H Vincenzo Librandi, <a href="/A007583/b007583.txt">Table of n, a(n) for n = 0..170</a>

%H David Applegate, Omar E. Pol and N. J. A. Sloane, <a href="http://neilsloane.com/doc/tooth.pdf">The Toothpick Sequence and Other Sequences from Cellular Automata</a>

%H C. Bebeacua, T. Mansour, A. Postnikov and S. Severini, <a href="http://arXiv.org/abs/math.CO/0506334">On the x-rays of permutations</a>, arXiv:math/0506334 [math.CO], 2005.

%H J. R. Britnell, M. Wildon, <a href="http://arxiv.org/abs/1507.04803">Bell numbers, partition moves and the eigenvalues of the random-to-top shuffle in types A, B and D</a>, arXiv:1507.04803 [math.CO], 2015.

%H E. Estrada and J. A. de la Pena, <a href="http://arxiv.org/abs/1302.1176">From Integer Sequences to Block Designs via Counting Walks in Graphs</a>, arXiv preprint arXiv:1302.1176 [math.CO], 2013. - From _N. J. A. Sloane_, Feb 28 2013

%H E. Estrada and J. A. de la Pena, <a href="http://www.nntdm.net/papers/nntdm-19/NNTDM-19-3-78-84.pdf">Integer sequences from walks in graphs</a>, Notes on Number Theory and Discrete Mathematics, Vol. 19, 2013, No. 3, 78-84

%H S. Hong and J. H. Kwak, <a href="http://dx.doi.org/10.1002/jgt.3190170509">Regular fourfold coverings with respect to the identity automorphism</a>, J. Graph Theory, 17 (1993), 621-627.

%H INRIA Algorithms Project, <a href="http://algo.inria.fr/ecs/ecs?searchType=1&amp;service=Search&amp;searchTerms=893">Encyclopedia of Combinatorial Structures 893</a>

%H W. Lang, <a href="http://arxiv.org/abs/1404.2710">On Collatz' Words, Sequences and Trees</a>, arXiv preprint arXiv:1404.2710 [math.NT], 2014.

%H Mircea Merca, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL15/Merca1/merca6.html">A Note on Cosine Power Sums</a> J. Integer Sequences, Vol. 15 (2012), Article 12.5.3.

%H Quynh Nguyen, Jean Pedersen, and Hien T. Vu, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL19/Pedersen/pedersen2.html">New Integer Sequences Arising From 3-Period Folding Numbers</a>, Vol. 19 (2016), Article 16.3.1.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Repunit.html">Repunit</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (5,-4).

%F a(n) = 2*A002450(n) + 1.

%F a(n) = Sum_{m=0..n} A060920(n, m) = A002450(n+1)-2*A002450(n). G.f.: (1-2*x)/(1-5*x+4*x^2). - _Wolfdieter Lang_, Apr 24 2001

%F a(n) = Sum_{k=0..n} binomial(n+k, 2*k)/2^(k-n). a(n)= 4*a(n-1)-1, n>0.

%F a(n) = 1 + 2*Sum_{k=0..n-1} 4^k; a(n) = A001045(2n+1). - _Paul Barry_, Mar 17 2003

%F a(n) = A020988(n-1)+1 = A039301(n+1)-1 = A083584(n-1)+2. - _Ralf Stephan_, Jun 14 2003

%F a(0) = 1; a(n+1) = a(n) * 4 - 1. - Regis Decamps (decamps(AT)users.sf.net), Feb 04 2004

%F a(n) = Sum_{i+j+k=n; 0 <= i,j,k <= n} (n+k)!/i!/j!/(2*k)!. - _Benoit Cloitre_, Mar 25 2004

%F a(n) = 5*a(n-1)-4*a(n-2). - _Emeric Deutsch_, Apr 01 2004

%F a(n) = 4^n-A001045(2*n). - _Paul Barry_, Apr 17 2004

%F a(n) = 2*(A001045(n))^2+(A001045(n+1))^2. - _Paul Barry_, Jul 15 2004

%F a(n) = left and right terms in M^n * [1 1 1] where M = the 3X3 matrix [1 1 1 / 1 3 1 / 1 1 1]. M^n * [1 1 1] = [a(n) A002450(n+1) a(n)] E.g. a(3) = 43 since M^n * [1 1 1] = [43 85 43] = [a(3) A002450(4) a(3)]. - _Gary W. Adamson_, Dec 18 2004

%F a(n) = A072197(n) - A020988(n). - _Creighton Dement_, Dec 31 2004

%F a(n) = A139250(2^n). - _Omar E. Pol_, Feb 28 2011

%F a(n) = A193652(2*n+1). - _Reinhard Zumkeller_, Aug 08 2011

%F a(n) = Sum_{k=-floor(n/3)..floor(n/3)} binomial(2*n,n+3*k)/2. - _Mircea Merca_, Jan 28 2012

%F a(n) = 2^(2*(n+1)) - A072197(n). - _Vladimir Pletser_, Apr 12 2014

%F a(n) == 2*n+1 (mod 3). Indeed, from Regis Decamps' formula (Feb 04 2004) we have a(i+1) - a(i) == -1 (mod 3), i= 0,1,...,n-1. Summing, we have a(n) - 1 == -n (mod 3), and the formula follows. - _Vladimir Shevelev_, May 13, 20 2015

%F For n>0 a(n) = A133494(0) + 2 * (A133494(n) + Sum_{x=1..n-1}Sum_{k=0..x-1}(binomial(x-1,k)*(A133494(k+1)+A133494(n-x+k)))). - _J. Conrad_, Dec 06 2015

%F a(n) = Sum_{k=0..2n} (-2)^k == 1 + Sum_{k=1..n} 2^(2k-1). - _Bob Selcoe_, Aug 21 2016

%F E.g.f.: (1 + 2*exp(3*x))*exp(x)/3. - _Ilya Gutkovskiy_, Aug 21 2016

%p a[0]:=1:for n from 1 to 50 do a[n]:=4*a[n-1]-1 od: seq(a[n], n=0..23); # _Zerinvary Lajos_, Feb 22 2008

%p A007583 := proc(n)

%p (2^(2*n+1)+1)/3 ;

%p end proc: # _R. J. Mathar_, Feb 19 2015

%t Table[(2^(2 n + 1) + 1)/3, {n, 0, 23}] (* or *)

%t Table[1 + 2 Sum[4^k, {k, 0, n - 1}], {n, 0, 23}] (* or *)

%t NestList[4 # - 1 &, 1, 23] (* or *)

%t Table[Sum[Binomial[n + k, 2 k]/2^(k - n), {k, 0, n}], {n, 0, 23}] (* or *)

%t CoefficientList[Series[(1 - 2 x)/(1 - 5 x + 4 x^2), {x, 0, 23}], x] (* _Michael De Vlieger_, Aug 22 2016 *)

%o (PARI) a(n)=sum(k=-n\3,n\3,binomial(2*n+1,n+1+3*k))

%o (PARI) a=1;for(n=1,23,print1(a,", ");a=bitor(a,3*a)) \\ _K. Spage_, Aug 20 2014

%o (PARI) Vec((1-2*x)/(1-5*x+4*x^2) + O(x^100)) \\ _Altug Alkan_, Dec 08 2015

%o (MAGMA) [(2^(2*n+1) + 1)/3: n in [0..40] ]; // _Vincenzo Librandi_, Apr 28 2011

%o (Haskell)

%o a007583 = (`div` 3) . (+ 1) . a004171

%o -- _Reinhard Zumkeller_, Jan 09 2013

%Y Cf. also A006054, A006356, A005578.

%Y Partial sums of A081294.

%Y Cf. A002450, A004171, A007051, A083065, A083066, A083884.

%Y Cf. A000979, A000978, A124400, A124401, A127955, A127956, A127957, A127958, A127936.

%K nonn,easy

%O 0,2

%A _Simon Plouffe_

%E Minor corrections to Maple program and fifth formula concerning the lead index by _K. Spage_, Aug 20 2014

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Last modified December 2 23:29 EST 2016. Contains 278694 sequences.