

A001045


Jacobsthal sequence (or Jacobsthal numbers): a(n) = a(n1) + 2*a(n2), with a(0) = 0, a(1) = 1; also a(n) = nearest integer to 2^n/3.
(Formerly M2482 N0983)


700



0, 1, 1, 3, 5, 11, 21, 43, 85, 171, 341, 683, 1365, 2731, 5461, 10923, 21845, 43691, 87381, 174763, 349525, 699051, 1398101, 2796203, 5592405, 11184811, 22369621, 44739243, 89478485, 178956971, 357913941, 715827883, 1431655765, 2863311531, 5726623061, 11453246123
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OFFSET

0,4


COMMENTS

Don Knuth points out (personal communication) that Jacobsthal may never have seen the actual values of this sequence. However, Horadam uses the name "Jacobsthal sequence", such an important sequence needs a name, and there is a law that says the name for something should never be that of its discoverer.  N. J. A. Sloane, Dec 26 2020
Number of ways to tile a 3 X (n1) rectangle with 1 X 1 and 2 X 2 square tiles.
Also, number of ways to tile a 2 X (n1) rectangle with 1 X 2 dominoes and 2 X 2 squares.  Toby Gottfried, Nov 02 2008
Also a(n) counts each of the following four things: nary quasigroups of order 3 with automorphism group of order 3, nary quasigroups of order 3 with automorphism group of order 6, (n1)ary quasigroups of order 3 with automorphism group of order 2 and (n2)ary quasigroups of order 3. See the McKayWanless (2008) paper.  Ian Wanless, Apr 28 2008
Also the number of ways to tie a necktie using n + 2 turns. So three turns make an "oriental", four make a "four in hand" and for 5 turns there are 3 methods: "Kelvin", "Nicky" and "Pratt". The formula also arises from a special random walk on a triangular grid with side conditions (see Fink and Mao, 1999).  arne.ring(AT)epost.de, Mar 18 2001
Also the number of compositions of n + 1 ending with an odd part (a(2) = 3 because 3, 21, 111 are the only compositions of 3 ending with an odd part). Also the number of compositions of n + 2 ending with an even part (a(2) = 3 because 4, 22, 112 are the only compositions of 4 ending with an even part).  Emeric Deutsch, May 08 2001
Arises in study of sorting by merge insertions and in analysis of a method for computing GCDs  see Knuth reference.
Number of perfect matchings of a 2 X n grid upon replacing unit squares with tetrahedra (C_4 to K_4):
oooo...
 \/  \/  \/ 
 /\  /\  /\ 
Also the numerators of the reduced fractions in the alternating sum 1/2  1/4 + 1/8  1/16 + 1/32  1/64 + ...  Joshua Zucker, Feb 07 2002
Also, if A(n), B(n), C(n) are the angles of the northic triangle of ABC then A(1) = Pi  2*A, A(n) = s(n)*Pi + (2)^n*A where s(n) = (1)^(n1) * a(n) [1orthic triangle = the orthic triangle of ABC, northic triangle = the orthic triangle of the (n1)orthic triangle].  Antreas P. Hatzipolakis (xpolakis(AT)otenet.gr), Jun 05 2002
Also the number of words of length n+1 in the two letters s and t that reduce to the identity 1 by using the relations sss = 1, tt = 1 and stst = 1. The generators s and t and the three stated relations generate the group S3.  John W. Layman, Jun 14 2002
Sums of pairs of consecutive terms give all powers of 2 in increasing order.  Amarnath Murthy, Aug 15 2002
Excess clockwise moves (over counterclockwise) needed to move a tower of size n to the clockwise peg is (1)^n*(2^n  (1)^n)/3; a(n) is its unsigned version.  Wouter Meeussen, Sep 01 2002
Also the absolute value of the number represented in base 2 by the string of n 1's, the negabinary repunit. The Mersenne numbers (A000225 and its subsequences) are the binary repunits.  Rick L. Shepherd, Sep 16 2002
Note that 3*a(n) + (1)^n = 2^n is significant for Pascal's triangle A007318. It arises from a Jacobsthal decomposition of Pascal's triangle illustrated by 1 + 7 + 21 + 35 + 35 + 21 + 7 + 1 = (7 + 35 + 1) + (1 + 35 + 7) + (21 + 21) = 43 + 43 + 42 = 3a(7)  1; 1 + 8 + 28 + 56 + 70 + 56 + 28 + 8 + 1 = (1 + 56 + 28) + (28 + 56 + 1) + (8 + 70 + 8) = 85 + 85 + 86 = 3a(8)+1.  Paul Barry, Feb 20 2003
Number of positive integers requiring exactly n signed bits in the nonadjacent form representation.
Equivalently, number of length(n1) words with letters {0, 1, 2} where no two consecutive letters are nonzero, see example and fxtbook link.  Joerg Arndt, Nov 10 2012
Counts walks between adjacent vertices of a triangle.  Paul Barry, Nov 17 2003
Every amphichiral rational knot written in Conway notation is a palindromic sequence of numbers, not beginning or ending with 1. For example, for 4 <= n <= 12, the amphichiral rational knots are: 2 2, 2 1 1 2, 4 4, 3 1 1 3, 2 2 2 2, 4 1 1 4, 3 1 1 1 1 3, 2 3 3 2, 2 1 2 2 1 2, 2 1 1 1 1 1 1 2, 6 6, 5 1 1 5, 4 2 2 4, 3 3 3 3, 2 4 4 2, 3 2 1 1 2 3, 3 1 2 2 1 3, 2 2 2 2 2 2, 2 2 1 1 1 1 2 2, 2 1 2 1 1 2 1 2, 2 1 1 1 1 1 1 1 1 2. For the number of amphichiral rational knots for n=2*k (k=1, 2, 3, ...), we obtain the sequence 0, 1, 1, 3, 5, 11, 21, 43, 85, 171, 341, 683, ...  Slavik Jablan, Dec 26 2003
a(n+2) counts the binary sequences of total length n made up of codewords from C = {0, 10, 11}.  Paul Barry, Jan 23 2004
Number of permutations with no fixed points avoiding 231 and 132.
The nth entry (n > 1) of the sequence is equal to the 2,2entry of the nth power of the unnormalized 4 X 4 Haar matrix: [1 1 1 0 / 1 1 1 0 / 1 1 0 1 / 1 1 0 1].  Simone Severini, Oct 27 2004
a(n) is the number of Motzkin (n+1)sequences whose flatsteps all occur at level 1 and whose height is less than or equal to 2. For example, a(4) = 5 counts UDUFD, UFDUD, UFFFD, UFUDD, UUDFD.  David Callan, Dec 09 2004
If (m + n) is odd, then 3*(a(m) + a(n)) is always of the form a^2 + 2*b^2, where a and b both equal powers of 2; consequently every factor of (a(m) + a(n)) is always of the form a^2 + 2*b^2.  Matthew Vandermast, Jul 12 2003
Number of "0,0" in f_{n+1}, where f_0 = "1" and f_{n+1} = a sequence formed by changing all "1"s in f_n to "1,0" and all "0"s in f_n to "0,1".  Fung Cheok Yin (cheokyin_restart(AT)yahoo.com.hk), Sep 22 2006
All prime Jacobsthal numbers A049883[n] = {3, 5, 11, 43, 683, 2731, 43691, ...} have prime indices except for a(4) = 5. All prime Jacobsthal numbers with prime indices (all but a(4) = 5) are of the form (2^p + 1)/3  the Wagstaff primes A000979[n]. Indices of prime Jacobsthal numbers are listed in A107036[n] = {3, 4, 5, 7, 11, 13, 17, 19, 23, 31, 43, 61, ...}. For n>1 A107036[n] = A000978[n] Numbers n such that (2^n + 1)/3 is prime.  Alexander Adamchuk, Oct 03 2006
Correspondence: a(n) = b(n)*2^(n1), where b(n) is the sequence of the arithmetic means of previous two terms defined by b(n) = 1/2*(b(n1) + b(n2)) with initial values b(0) = 0, b(1) = 1; the g.f. for b(n) is B(x) := x/(1(x^1+x^2)/2), so the g.f. A(x) for a(n) satisfies A(x) = B(2*x)/2. Because b(n) converges to the limit lim (1x)*B(x) = 1/3*(b(0) + 2*b(1)) = 2/3 (for x > 1), it follows that a(n)/2^(n1) also converges to 2/3 (see also A103770).  Hieronymus Fischer, Feb 04 2006
Inverse: floor(log_2(a(n)) = n  2 for n >= 2. Also: log_2(a(n) + a(n1)) = n  1 for n >= 1 (see also A130249). Characterization: x is a Jacobsthal number if and only if there is a power of 4 (= c) such that x is a root of p(x) = 9*x*(xc) + (c1)*(2*c+1) (see also the indicator sequence A105348).  Hieronymus Fischer, May 17 2007
This sequence counts the odd coefficients in the expansion of (1 + x + x^2)^(2^n  1), n >= 0.  Tewodros Amdeberhan (tewodros(AT)math.mit.edu), Oct 18 2007, Jan 08 2008
2^(n+1) = 2*A005578(n) + 2*a(n) + 2*A000975(n1). Let A005578(n), a(n), A000975(n1) = triangle (a, b, c). Then ((Sc), (Sb), (Sa)) = (A005578(n1), a(n1), A000975(n2)). Example: (a, b, c) = (11, 11, 10) = (A005578(5), a(5), A000975(4). Then ((Sc), (Sb), (Sa)) = (6, 5, 5) = (A005578(4), a(4), A000975(3)).  Gary W. Adamson, Dec 24 2007
Sequence is identical to the absolute values of its inverse binomial transform. A similar result holds for [0,A001045*2^n].  Paul Curtz, Jan 17 2008
From a(2) on (i.e., 1, 3, 5, 11, 21, ...) also: least odd number such that the subsets of {a(2), ..., a(n)} sum to 2^(n1) different values, cf. A138000 and A064934. It is interesting to note the pattern of numbers occurring (or not occurring) as such a sum (A003158).  M. F. Hasler, Apr 09 2008
It appears that a(n) is also the number of integers between 2^n and 2^(n+1) that are divisible by 3 with no remainder.  John Fossaceca (john(AT)fossaceca.net), Jan 31 2009
Number of pairs of consecutive odious (or evil) numbers between 2^(n+1) and 2^(n+2), inclusive.  T. D. Noe, Feb 05 2009
A threedimensional interpretation of a(n+1) is that it gives the number of ways of filling a 2 X 2 X n hole with 1 X 2 X 2 bricks.  Martin Griffiths, Mar 28 2009
Convolved with (1, 2, 2, 2, ...) = A000225: (1, 3, 7, 15, 31, ...).  Gary W. Adamson, May 23 2009
The product of a pair of successive terms is always a triangular number.  Giuseppe Ottonello, Jun 14 2009
Let A be the Hessenberg matrix of order n, defined by: A[1, j] = 1, A[i, i] := 2, A[i, i  1] = 1, and A[i, j] = 0 otherwise. Then, for n >= 1, a(n) = (1)^(n1)*det(A).  Milan Janjic, Jan 26 2010
Let R denote the irreducible representation of the symmetric group S_3 of dimension 2, and let s and t denote respectively the sign and trivial irreducible representations of dimension 1. The decomposition of R^n into irreducible representations consists of a(n) copies of R and a(n1) copies of each of s and t.  Andrew Rupinski, Mar 12 2010
As a fraction: 1/88 = 0.0113636363... or 1/9898 = 0.00010103051121...  Mark Dols, May 18 2010
Starting with "1" = the INVERT transform of (1, 0, 2, 0, 4, 0, 8, ...); e.g., a(7) = 43 = (1, 1, 1, 3, 5, 11, 21) dot (8, 0, 4, 0, 2, 0, 1) = (8 + 4 + 10 + 21) = 43.  Gary W. Adamson, Oct 28 2010
Rule 28 elementary cellular automaton (A266508) generates this sequence.  Paul Muljadi, Jan 27 2011
Let U be the unitprimitive matrix (see [Jeffery])
U = U_(6,2) =
(0 0 1)
(0 2 0)
(2 0 1).
Then a(n+1) = (Trace(U^n))/3, a(n+1) = ((U^n)_{3, 3})/3, a(n) = ((U^n)_{1, 3})/3 and a(n) = ((U^(n+1))_{1, 1})/2. (End)
The sequence emerges in using iterated deletion of strictly dominated strategies to establish the bestresponse solution to the Cournot duopoly problem as a strictly dominant strategy. The best response of firm 1 to firm 2's chosen quantity is given by q*_1 = 1/2*(a  c  q_2), where a is reservation price, c is marginal cost, and q_2 is firm two's chosen quantity. Given that q_2 is in [o, a  c], q*_1 must be in [o, 1/2*(a  c)]. Since costs are symmetric, we know q_2 is in [0, 1/2*(a  c)]. Then we know q*_1 is in [1/4*(a  c), 1/2*(a  c)]. Continuing in this way, the sequence of bounds we get (factoring out a  c) is {1/2, 1/4, 3/8, 5/16, ...}; the numerators are the Jacobsthal numbers.  Michael Chirico, Sep 10 2011
The compositions of n in which each natural number is colored by one of p different colors are called pcolored compositions of n. For n >= 2, 3*a(n1) equals the number of 3colored compositions of n with all parts greater than or equal to 2, such that no adjacent parts have the same color.  Milan Janjic, Nov 26 2011
This sequence is connected with the Collatz problem. We consider the array T(i, j) where the ith row gives the parity trajectory of i, for example for i = 6, the infinite trajectory is 6 > 3 > 10 > 5 > 16 > 8 > 4 > 2 > 1 > 4 > 2 > 1 > 4 > 2 > 1... and T(6, j) = [0, 1, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, ..., 1, 0, 0, 1, ...]. Now, we consider the sum of the digits "1" of each column. We obtain the sequence a(n) = Sum_{k = 1..2^n} T(k, n) = Sum _{k = 1..2^n} digits "1" of the nth column. Because a(n) + a(n+1) = 2^n, then a(n+1) = Number of digits "0" among the 2^n elements of the nth column.  Michel Lagneau, Jan 11 2012
3!*a(n1) is apparently the trace of the nth power of the adjacency matrix of the complete 3graph, a 3 X 3 matrix with diagonal elements all zero and offdiagonal all ones. The offdiagonal elements for the nth power are all equal to a(n) while each diagonal element seems to be a(n) + 1 for an even power and a(n)  1 for an odd. These are related to the lengths of closed paths on the graph (see Delfino and Viti's paper).  Tom Copeland, Nov 06 2012
2^n * a(n) = (1)^(n1) * a(n), which extends the sequence to negative indices: ..., 5/16, 3/8, 1/4, 1/2, 0, 1, 1, 3, 5, ...
The "autosequence" property with respect to the binomial transform mentioned in my comment of Jan 17 2008 is still valid if the term a(1) is added to the array of the sequence and its iterated higherorder differences in subsequent rows:
0 1/2 1/2 3/2 5/2 11/2 ...
1/2 0 1 1 3 5 ...
1/2 1 0 2 2 6 ...
3/2 1 2 0 4 4 ...
5/2 3 2 4 0 8 ...
11/2 5 6 4 8 0 ...
The main diagonal in this array contains 0's. (End)
Assign to a triangle T(n, 0) = 1 and T(n+1, 1) = n; T(r, c) = T(r1, c1) + T(r1, c2) + T(r2, c2). Then T(n+1, n)  T(n, n) = a(n).  J. M. Bergot, May 02 2013
a(n+1) counts clockwise walks on n points on a circle that take steps of length 1 and 2, return to the starting point after two full circuits, and do not duplicate any steps (USAMO 2013, problem 5).  Kiran S. Kedlaya, May 11 2013
Define an infinite square array m by m(n, 0) = m(0, n) = a(n) in top row and left column and m(i, j) = m(i, j1) + m(i1, j1) otherwise, then m(n+1, n+1) = 3^(n1).  J. M. Bergot, May 10 2013
a(n) is the number of compositions (ordered partitions) of n  1 into one sort of 1's and two sorts of 2's. Example: the a(4) = 5 compositions of 3 are 1 + 1 + 1, 1 + 2, 1 + 2', 2 + 1 and 2' + 1.  Bob Selcoe, Jun 24 2013
Without 0, a(n)/2^n equals the probability that n will occur as a partial sum in a randomlygenerated infinite sequence of 1's and 2's. The limiting ratio is 2/3.  Bob Selcoe, Jul 04 2013
Number of conjugacy classes of Z/2Z X Z/2Z in GL(2,2^(n+1)).  _Jared Warner_, Aug 18 2013
a(n) is the top left entry of the (n1)st power of the 3 X 3 matrix [1, 1, 1, 1, 0, 0, 1, 0, 0]. a(n) is the top left entry of the (n+1)st power of any of the six 3 X 3 matrices [0, 1, 0; 1, 1, 1; 0, 1, 0], [0, 1, 1; 0, 1, 1; 1, 1, 0], [0, 0, 1; 1, 1, 1; 1, 1, 0], [0, 1, 1; 1, 0, 1; 0, 1, 1], [0, 0, 1; 0, 0, 1; 1, 1, 1] or [0, 1, 0; 1, 0, 1; 1, 1, 1].  R. J. Mathar, Feb 03 2014
This is the only integer sequence from the family of homogeneous linear recurrence of order 2 given by a(n) = k*a(n1) + t*a(n2) with positive integer coefficients k and t and initial values a(0) = 0 and a(1) = 1 whose ratio a(n+1)/a(n) converges to 2 as n approaches infinity.  Felix P. Muga II, Mar 14 2014
sqrt(a(n+1) * a(n1)) > a(n) + 3/4 if n is even, and > a(n)  3/4 if n is odd, for n >= 2.  Richard R. Forberg, Jun 24 2014
a(n+1) counts closed walks on the end vertices of P_3 containing one loop at the middle vertex. a(n1) counts closed walks on the middle vertex of P_3 containing one loop at that vertex.  David Neil McGrath, Nov 07 2014
Let P be a point in the plane of a triangle ABC (with sides a, b, c) and barycentric coordinates P = [x:y:z]. The Complement of P with respect to ABC is defined to be Complement(P) = [b*y + c*z : c*z + a*x : a*x + b*y].
Then, for n >= 1, Complement(Complement(...(Complement(P))..)) = (n times) =
[2*a(n1)*a*x + (2*a(n1)  (1)^n)*(b*y + c*z):
2*a(n1)*b*y + (2*a(n1)  (1)^n)*(c*z + a*x):
2*a(n1)*c*z + (2*a(n1)  (1)^n)*(a*x + b*y)]. (End)
a(n) (n >= 2) is the number of induced hypercubes of the Fibonacci cube Gamma(n2). See p. 513 of the Klavzar reference. Example: a(5) = 11. Indeed, the Fibonacci cube Gamma(3) is <> (cycle C(4) with a pendant edge) and the hypercubes are: 5 vertices, 5 edges, and 1 square.  Emeric Deutsch, Apr 07 2016
If the sequence of points {P_i(x_i, y_i)} on the cubic y = a*x^3 + b*x^2 + c*x + d has the property that the segment P_i(x_i, y_i) P_i+1(x_i+1, y_i+1) is always tangent to the cubic at P_i+1(x_i+1, y_i+1) then a(n) = 2^n*a/b*(x_(n+1)(1/2)^n*x_1).  Michael Brozinsky, Aug 01 2016
With the quantum integers defined by [n+1]_q = (q^(n+1)  q^(n1)) / (q  q^(1)), the Jacobsthal numbers are a(n+1) = (1)^n*q^n [n+1]_q with q = i * sqrt(2) for i^2 = 1, whereas the signed Mersenne numbers A000225 are given by q = sqrt(2). Cf. A239473.  Tom Copeland, Sep 05 2016
Every positive integer has a unique expression as a sum of Jacobsthal numbers in which the index of the smallest summand is odd, with a(1) and a(2) both allowed. See the L. Carlitz, R. Scoville, and V. E. Hoggatt, Jr. reference.  Ira M. Gessel, Dec 31 2016. See A280049 for these expansions.  N. J. A. Sloane, Dec 31 2016
For n > 0, a(n) equals the number of ternary words of length n1 in which 0 and 1 avoid runs of odd lengths.  Milan Janjic, Jan 08 2017
For n > 0, a(n) equals the number of orbits of the finite group PSL(2,2^n) acting on subsets of size 4 for the 2^n+1 points of the projective line.  Paul M. Bradley, Jan 31 2017
For n > 1, number of words of length n2 over alphabet {1,2,3} such that no odd letter is followed by an odd letter.  Armend Shabani, Feb 17 2017
Also, the decimal representation of the xaxis, from the origin to the right edge, of the nth stage of growth of the twodimensional cellular automaton defined by "Rule 678", based on the 5celled von Neumann neighborhood, initialized with a single black (ON) cell at stage zero. See A283641.  Robert Price, Mar 12 2017
Also the number of independent vertex sets and vertex covers in the 2 X (n2) king graph.  Eric W. Weisstein, Sep 21 2017
Let T(0) be a triangle and let T(1) be the medial triangle of T(0), T(2) the medial triangle of T(1) and, in general, T(n) the medial triangle of T(n1). The barycentric coordinates of the first vertex of T(n) are [2*a(n1)/a(n), 1, 1], for n > 0.
Let S(0) be a triangle and let S(1) be the antimedial triangle of S(0), S(2) the antimedial triangle of S(1) and, in general, S(n) the antimedial triangle of S(n1). The barycentric coordinates of the first vertex of S(n) are [a(n+1)/a(n), 1, 1], for n > 0. (End)
a(n) is also the number of derangements in S_{n+1} with empty peak set.  Isabella Huang, Apr 01 2018
For n > 0, gcd(a(n), a(n+1)) = 1.  Kengbo Lu, Jul 27 2020
Number of 2compositions of n+1 with 1 not allowed as a part; see Hopkins & Ouvry reference.  Brian Hopkins, Aug 17 2020
The number of Hamiltonian paths of the flower snark graph of even order 2n > 2 is 12*a(n1).  Don Knuth, Dec 25 2020
When set S = {1, 2, ..., 2^n}, n>=0, then the largest subset T of S with the property that if x is in T, then 2*x is not in T, has a(n+1) elements. For example, for n = 4, #S = 16, a(5) = 11 with T = {1, 3, 4, 5, 7, 9, 11, 12, 13, 15, 16} (see Hassan Tarfaoui link, Concours Général 1991).  Bernard Schott, Feb 14 2022
a(n) is the number of words of length n over a binary alphabet whose position in the lexicographic order is one more than a multiple of three. a(3) = 3: aaa, abb, bba.  Alois P. Heinz, Apr 13 2022
Named by Horadam (1988) after the German mathematician Ernst Jacobsthal (18821965).  Amiram Eldar, Oct 02 2023
Define the sequence u(n) = (u(n1) + u(n2))/u(n3) with u(0) = 0, u(1) = 1, u(2) = u(3) = 1. Then u(4*n) = 1 + (1)^n/a(n+1), u(4*n+1) = 2  (1)^n/a(n+1), u(4*n+2) = u(4*n+3) = 1. For example, a(3) = 3 and u(8) = 2/3, u(9) = 5/3, u(10) = u(11) = 1.  Michael Somos, Oct 24 2023


REFERENCES

Jathan Austin and Lisa Schneider, Generalized Fibonacci sequences in Pythagorean triple preserving sequences, Fib. Q., 58:1 (2020), 340350.
Thomas Fink and Yong Mao, The 85 ways to tie a tie, Fourth Estate, London, 1999; Die 85 Methoden eine Krawatte zu binden. Hoffmann und Kampe, Hamburg, 1999.
International Mathematical Olympiad 2001, Hong Kong Preliminary Selection Contest Problem #16.
Jablan S. and Sazdanovic R., LinKnot: Knot Theory by Computer, World Scientific Press, 2007. See p. 80.
Ernst Erich Jacobsthal, Fibonaccische Polynome und Kreisteilungsgleichungen, Sitzungsber. Berliner Math. Gesell. 17 (19191920), 4357.
Tanya Khovanova, "Coins and Logic", Chapter 6, The Mathematics of Various Entertaining Subjects: Volume 3 (2019), Jennifer Beineke & Jason Rosenhouse, eds. Princeton University Press, Princeton and Oxford, p. 73. ISBN: 0691182582, 9780691182582.
Donald E. Knuth, Art of Computer Programming, Vol. 3, Sect. 5.3.1, Eq. 13.
Thomas Koshy, Fibonacci and Lucas numbers with applications, Wiley, 2001, p. 98.
Steven Roman, Introduction to Coding and Information Theory, Springer Verlag, 1996, 4142.
P. D. Seymour and D. J. A. Welsh, Combinatorial applications of an inequality form statistical mechanics, Math. Proc. Camb. Phil. Soc. 77 (1975), 485495. [Although Daykin et al. (1979) claim that the present sequence is studied in this article, it does not seem to be explicitly mentioned. Note that definition of logconvex in (3.1) is wrong.  N. J. A. Sloane, Dec 26 2020]
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Robert M. Young, Excursions in Calculus, MAA, 1992, p. 239


LINKS

Paula Catarino, Helena Campos, and Paulo Vasco. On the Mersenne sequence. Annales Mathematicae et Informaticae, 46 (2016) pp. 3753.
S. Northshield, Stern's diatomic sequence 0, 1, 1, 2, 1, 3, 2, 3, 1, 4, ..., Amer. Math. Monthly, 117 (2010), 581598.
USA Mathematical Olympiad 2013, Problem 2 (proposed by Sam Vandervelde).
Eric Weisstein's World of Mathematics, King Graph.
G. B. M. Zerr, Problem 64, American Mathematical Monthly, vol. 3, no. 12, 1896 (p. 311).


FORMULA

a(n) = 2^(n1)  a(n1). a(n) = 2*a(n1)  (1)^n = (2^n  (1)^n)/3.
G.f.: x/(1  x  2*x^2).
E.g.f.: (exp(2*x)  exp(x))/3.
a(2*n) = 2*a(2*n1)1 for n >= 1, a(2*n+1) = 2*a(2*n)+1 for n >= 0.  Lee Haehwang, Oct 11 2002; corrected by Mario Catalani (mario.catalani(AT)unito.it), Dec 04 2002
Also a(n) is the coefficient of x^(n1) in the bivariate Fibonacci polynomials F(n)(x, y) = x*F(n1)(x, y) + y*F(n2)(x, y), with y=2*x^2.  Mario Catalani (mario.catalani(AT)unito.it), Dec 04 2002
a(n) = Sum_{k=1..n} binomial(n, k)*(1)^(n+k)*3^(k1).  Paul Barry, Apr 02 2003
The ratios a(n)/2^(n1) converge to 2/3 and every fraction after 1/2 is the arithmetic mean of the two preceding fractions.  Gary W. Adamson, Jul 05 2003
a(n) = U(n1, i/(2*sqrt(2)))*(i*sqrt(2))^(n1) with i^2=1.  Paul Barry, Nov 17 2003
a(n+1) = Sum_{k=0..ceiling(n/2)} 2^k*binomial(nk, k).  Benoit Cloitre, Mar 06 2004
a(n) = round(2^n/3) = (2^n + (1)^(n1))/3 so lim_{n>infinity} 2^n/a(n) = 3.  Gerald McGarvey, Jul 21 2004
a(n) = Sum_{k=0..n1} (1)^k*2^(nk1) = Sum_{k=0..n1}, 2^k*(1)^(nk1).  Paul Barry, Jul 30 2004
a(n+1) = Sum_{k=0..n} binomial(k, nk)*2^(nk).  Paul Barry, Oct 07 2004
a(n) = Sum_{k=0..n1} W(nk, k)*(1)^(nk)*binomial(2*k,k), W(n, k) as in A004070.  Paul Barry, Dec 17 2004
a(n) = Sum_{k=0..n} k*binomial(n1, (nk)/2)*(1+(1)^(n+k))*floor((2*k+1)/3).
a(n+1) = Sum_{k=0..n} k*binomial(n1, (nk)/2)*(1+(1)^(n+k))*(A042965(k)+0^k). (End)
a(n+1) = ceiling(2^n/3) + floor(2^n/3) = (ceiling(2^n/3))^2  (floor(2^n/3))^2.
a(n+1) = Sum_{k=0..n} Sum_{j=0..n} (1)^(nj)*binomial(j, k).  Paul Barry, Jan 26 2005
Let M = [1, 1, 0; 1, 0, 1; 0, 1, 1], then a(n) = (M^n)[2, 1], also matrix characteristic polynomial x^3  2*x^2  x + 2 defines the threestep recursion a(0)=0, a(1)=1, a(2)=1, a(n) = 2*a(n1) + a(n2)  2*a(n3) for n > 2.  Lambert Klasen (lambert.klasen(AT)gmx.net), Jan 28 2005
a(n) = Sum_{k=0..floor(n/3)} binomial(n, f(n1)+3*k);
a(n) = Sum_{k=0..floor(n/3)} binomial(n, f(n2)+3*k), where f(n)=A080425(n). (End)
a(2*n) = (1/3)*Product_{dn} cyclotomic(d,4).
a(2*n+1) = (1/3)*Product_{d2*n+1} cyclotomic(2*d,2). (End)
The a(n) are closely related to nested square roots; this is 2*sin(2^(n)*Pi/2*a(n)) = sqrt(2sqrt(2sqrt(2sqrt(...sqrt(2)))...) {with the '2' n times, n >= 0}.
Also 2*cos(2^(n)*Pi*a(n)) = sqrt(2sqrt(2sqrt(2sqrt(...sqrt(2)))...) {with the '2' n1 times, n >= 1} as well as
2*sin(2^(n)*3/2*Pi*a(n)) = sqrt(2+sqrt(2+sqrt(2+sqrt(...sqrt(2)))...) {with the '2' n times, n >= 0} and
2*cos(2^(n)*3*Pi*a(n)) = sqrt(2+sqrt(2+sqrt(2+sqrt(...sqrt(2)))...) {with the '2' n1 times, n >= 1}.
a(n) = 2^(n+1)/Pi*arcsin(b(n+1)/2) where b(n) is defined recursively by b(0)=2, b(n)=sqrt(2b(n1)).
There is a similar formula regarding the arccos function, this is a(n) = 2^n/Pi*arccos(b(n)/2).
With respect to the sequence c(n) defined recursively by c(0)=2, c(n)=sqrt(2+c(n1)), the following formulas hold true: a(n) = 2^n/3*(1(1)^n*(12/Pi*arcsin(c(n+1)/2)); a(n) = 2^n/3*(1(1)^n*(11/Pi*arccos(c(n)/2)).
(End)
Let T = the 3 X 3 matrix [1,1,0; 1,0,1; 0,1,1]. Then T^n * [1,0,0,] = [A005578(n), a(n), A000975(n1)].  Gary W. Adamson, Dec 24 2007
a(n) = Sum_{k=1..n} K(2, k)*a(n  k)), where K(n,k) = k if 0 <= k <= n and K(n,k)=0 otherwise. (When using such a Kcoefficient, several different arguments to K or several different definitions of K may lead to the same integer sequence. For example, the Fibonacci sequence can be generated in several ways using the Kcoefficient.)  Thomas Wieder, Jan 13 2008
a(n) + a(n+2*k+1) = a(2*k+1)*2^n.  Paul Curtz, Feb 12 2008
a(n) = lower left term in the 2 X 2 matrix [0,2; 1,1]^n.  Gary W. Adamson, Mar 02 2008
a(n) = sqrt(8*a(n1)*a(n2) + 1). E.g., sqrt(3*5*8+1) = 11, sqrt(5*11*8+1) = 21.  Giuseppe Ottonello, Jun 14 2009
Let p[i] = Fibonacci(i1) and let A be the Hessenberg matrix of order n defined by: A[i,j] = p[ji+1], (i <= j), A[i,j] = 1, (i = j+1), and A[i,j] = 0 otherwise. Then, for n >= 1, a(n1) = det(A).  Milan Janjic, May 08 2010
Algebraically equivalent to replacing the 5's with 9's in the explicit (Binet) formula for the nth term in the Fibonacci sequence: The formula for the nth term in the Fibonacci sequence is F(n) = ((1+sqrt(5))^n  (1sqrt(5))^n)/(2^n*sqrt(5)). Replacing the 5's with 9's gives ((1+sqrt(9))^n  (1sqrt(9))^n)/(2^n*sqrt(9)) = (2^n+(1)^(n+1))/3 = (2^n(1)^(n))/3 = a(n).  Jeffrey R. Goodwin, May 27 2011
G.f.: x/(1x2*x^2) = G(0)/3; G(k) = 1  ((1)^k)/(2^k  2*x*4^k/(2*x*2^k  ((1)^k)/G(k+1))); (continued fraction 3 kind, 3step).
E.g.f.: G(0)/3; G(k) = 1  ((1)^k)/(2^k  2*x*4^k/(2*x*2^k  ((1)^k)*(k+1)/G(k+1))); (continued fraction 3rd kind, 3step). (End)
G.f.: Q(0)/3, where Q(k) = 1  1/(4^k  2*x*16^k/(2*x*4^k  1/(1 + 1/(2*4^k  8*x*16^k/(4*x*4^k + 1/Q(k+1)))))); (continued fraction).  Sergei N. Gladkovskii, May 21 2013
G.f.: Q(0)*x/2, where Q(k) = 1 + 1/(1  x*(2*k+1 + 2*x)/( x*(2*k+2 + 2*x) + 1/Q(k+1) )); (continued fraction).  Sergei N. Gladkovskii, Aug 29 2013
G.f.: Q(0) 1, where Q(k) = 1 + 2*x^2 + (k+2)*x  x*(k+1 + 2*x)/Q(k+1); (continued fraction).  Sergei N. Gladkovskii, Oct 06 2013
a(n) = (Sum_{k=1..n, k odd} C(n,k)*3^(k1))/2^(n1).  Vladimir Shevelev, Feb 05 2014
a(n) = (1)^n * a(n) / 2^n for all n in Z.  Michael Somos, Mar 18 2014
a(n) = (1)^(n1)*Sum_{k=0..n1} A135278(n1,k)*(3)^k = (2^n  (1)^n)/3 = (1)^(n1)*Sum_{k=0..n1} (2)^k. Equals (1)^(n1)*Phi(n,2), where Phi is the cyclotomic polynomial when n is an odd prime. (For n > 0.)  Tom Copeland, Apr 14 2014
a(2*n)/a(n) = A014551(n) for n >= 1; a(3*n)/a(n) = 3*A245489(n) for n >= 1.
exp( Sum_{n >= 1} a(2*n)/a(n)*x^n/n ) = Sum_{n >= 0} a(n+1)*x^n.
exp( Sum_{n >= 1} a(3*n)/a(n)*x^n/n ) = Sum_{n >= 0} A084175(n+1)*x^n.
exp( Sum_{n >= 1} a(4*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015266(n+3)*(x)^n.
exp( Sum_{n >= 1} a(5*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015287(n+4)*x^n.
exp( Sum_{n >= 1} a(6*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015305(n+5)*(x)^n.
exp( Sum_{n >= 1} a(7*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015323(n+6)*x^n.
exp( Sum_{n >= 1} a(8*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015338(n+7)*(x)^n.
exp( Sum_{n >= 1} a(9*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015356(n+8)*x^n.
exp( Sum_{n >= 1} a(10*n)/a(n)*x^n/n ) = Sum_{n >= 0} A015371(n+9)*(x)^n. (End)
Dirichlet g.f.: (PolyLog(s,2) + (1  2^(1s))*zeta(s))/3.  Ilya Gutkovskiy, Jun 27 2016
a(m)*a(n) + a(m1)*a(n1)  2*a(m2)*a(n2) = 2^(m+n3).
a(m+n1) = a(m)*a(n) + 2*a(m1)*a(n1); a(m+n) = a(m+1)*a(n+1)  4*a(m1)*a(n1).
a(2*n1) = a(n)^2 + 2*a(n1)^2; a(2*n) = a(n+1)^2  4*a(n1)^2. (End)
a(n+4) = a(n) + 5*2^n, a(0) = 0, a(1..4) = [1,1,3,5]. That is to say, for n > 0, the ones digits of Jacobsthal numbers follow the pattern 1,1,3,5,1,1,3,5,1,1,3,5,....  Yuchun Ji, Apr 25 2019
The sequence starting with "1" is the second INVERT transform of (1, 1, 3, 5, 11, 21, 43, ...).  Gary W. Adamson, Jul 08 2019
a(n)^2  a(n+1)*a(n1) = (2)^(n1).
a(n)^2  a(n+r)*a(nr) = (2)^(nr)*a(r)^2.
a(m)*a(n+1)  a(m+1)*a(n) = (2)^n*a(mn).
a(n) = Sum_{i=0..n1; j=0..n1; i+2*j=n1} 2^j*((i+j)!/(i!*j!)). (End)
For n > 0, 1/(2*a(n+1)) = Sum_{m>=n} a(m)/(a(m+1)*a(m+2)).  Kai Wang, Mar 03 2020
For 4 > h >= 0, k >= 0, a(4*k+h) mod 5 = a(h) mod 5.  Kai Wang, May 07 2020
a(n) = 1 + Sum_{k=0..n1} a(k) if n odd; a(n) = Sum_{k=0..n1} a(k) if n even.
a(n) = F(n) + Sum_{k=0..n2} a(k)*F(nk1), where F denotes the Fibonacci numbers.
a(n) = b(n) + Sum_{k=0..n1} a(k)*b(nk), where b(n) is defined through b(0) = 0, b(1) = 1, b(n) = 2*b(n2).
a(n) = 1 + 2*Sum_{k=0..n2} a(k).
a(m+n) = a(m)*a(n+1) + 2*a(m1)*a(n).
a(2*n) = Sum_{i>=0, j>=0} binomial(nj1,i)*binomial(ni1,j)*2^(i+j). (End)


EXAMPLE

a(2) = 3 because the tiling of the 3 X 2 rectangle has either only 1 X 1 tiles, or one 2 X 2 tile in one of two positions (together with two 1 X 1 tiles).
The a(6)=21 length5 ternary words with no two consecutive letters nonzero are (dots for 0's)
[ 1] [ . . . . ]
[ 2] [ . . . 1 ]
[ 3] [ . . . 2 ]
[ 4] [ . . 1 . ]
[ 5] [ . . 2 . ]
[ 6] [ . 1 . . ]
[ 7] [ . 1 . 1 ]
[ 8] [ . 1 . 2 ]
[ 9] [ . 2 . . ]
[10] [ . 2 . 1 ]
[11] [ . 2 . 2 ]
[12] [ 1 . . . ]
[13] [ 1 . . 1 ]
[14] [ 1 . . 2 ]
[15] [ 1 . 1 . ]
[16] [ 1 . 2 . ]
[17] [ 2 . . . ]
[18] [ 2 . . 1 ]
[19] [ 2 . . 2 ]
[20] [ 2 . 1 . ]
[21] [ 2 . 2 . ]
(End)
G.f. = x + x^2 + 3*x^3 + 5*x^4 + 11*x^5 + 21*x^6 + 43*x^7 + 85*x^8 + 171*x^9 + ...


MAPLE

(2^n(1)^n)/3 ;


MATHEMATICA

Jacob0[n_] := (2^n  (1)^n)/3; Table[Jacob0[n], {n, 0, 33}] (* Robert G. Wilson v, Dec 05 2005 *)
Array[(2^#  (1)^#)/3 &, 33, 0] (* Joseph Biberstine (jrbibers(AT)indiana.edu), Dec 26 2006 *)
LinearRecurrence[{1, 2}, {0, 1}, 40] (* Harvey P. Dale, Nov 30 2011 *)
CoefficientList[Series[x/(1  x  2 x^2), {x, 0, 34}], x] (* Robert G. Wilson v, Jul 21 2015 *)
Table[Abs[QBinomial[n, 1, 2]], {n, 0, 35}] (* John Keith, Jan 29 2022 *)


PROG

(PARI) a(n) = (2^n  (1)^n) / 3
(PARI) M=[1, 1, 0; 1, 0, 1; 0, 1, 1]; for(i=0, 34, print1((M^i)[2, 1], ", ")) \\ Lambert Klasen (lambert.klasen(AT)gmx.net), Jan 28 2005
(Sage) [lucas_number1(n, 1, 2) for n in range(34)] # Zerinvary Lajos, Apr 22 2009
# Alternatively:
a = BinaryRecurrenceSequence(1, 2)
(Haskell)
a001045 = (`div` 3) . (+ 1) . a000079
a001045_list = 0 : 1 :
zipWith (+) (map (2 *) a001045_list) (tail a001045_list)
(Maxima)
a[0]:0$
a[n]:=2^(n1)a[n1]$
(PARI) a=0; for(n=0, 34, print1(a, ", "); a=2*(an%2)+1) \\ K. Spage, Aug 22 2014
a, b = 0, 1
while True:
yield a
a, b = b, b+2*a
(Magma) [n le 2 select n1 else Self(n1)+2*Self(n2): n in [1..40]]; // Vincenzo Librandi, Jun 27 2016
(Python) [(2**n(1)**n)//3 for n in range(40)] # Gennady Eremin, Mar 03 2022


CROSSREFS

Partial sums of this sequence give A000975, where there are additional comments from B. E. Williams and Bill Blewett on the tie problem.
a(n) = A073370(n1, 0), n >= 1 (first column of triangle).
Cf. A000978, A000979, A019322, A066845, A105348, A130249, A130250, A130253, A005578, A002083, A113405, A138000, A064934, A003158, A175286 (Pisano periods), A147613, A156319, A002605, A000225, A052129, A014551 (companion "autosequence"), A015266, A015287, A015305, A015323, A015338, A015356, A015371, A084175, A245489, A283641.


KEYWORD

nonn,nice,easy,core,changed


AUTHOR



EXTENSIONS

Thanks to Don Knuth, who pointed out several missing references, including Brocard (1880), which although it was mentioned in the 1973 Handbook of Integer Sequences, was omitted from the 1995 "Encyclopedia".  N. J. A. Sloane, Dec 26 2020


STATUS

approved



