OFFSET
0,3
COMMENTS
This is the Gaussian binomial coefficient [n,1] for q=2.
Number of rank-1 matroids over S_n.
Numbers k such that the k-th central binomial coefficient is odd:A001405(k) mod 2 = 1. -Labos Elemer,Mar 12 2003
This gives the (zero-based) positions of odd terms in the following convolution sequences:A000108,A007460,A007461,A007463,A007464,A061922.
Also solutions (with minimum number of moves) for the problem of Benares Temple, i.e., three diamond needles with n discs ordered by decreasing size on the first needle to place in the same order on the third one, without ever moving more than one disc at a time and without ever placing one disc at the top of a smaller one. - Xavier Acloque, Oct 18 2003
a(0) = 0, a(1) = 1; a(n) = smallest number such that a(n)-a(m) == 0 (mod (n-m+1)), for all m. -Amarnath Murthy,Oct 23 2003
Binomial transform of [1, 1/2, 1/3,...] = [1/1, 3/2, 7/3,...]; (2^n - 1)/n, n=1,2,3,... -Gary W. Adamson,Apr 28 2005
Numbers whose binary representation is 111...1. E.g., the 7th term is (2^7) - 1 = 127 = 1111111 (in base 2). -Alexandre Wajnberg,Jun 08 2005
Number of nonempty subsets of a set with n elements. -Michael Somos,Sep 03 2006
For n >= 2, a(n) is the least Fibonacci n-step number that is not a power of 2. -Rick L. Shepherd,Nov 19 2007
Let P(A) be the power set of an n-element set A. Then a(n+1) = the number of pairs of elements {x,y} of P(A) for which x and y are disjoint and for which either x is a subset of y or y is a subset of x. -Ross La Haye,Jan 10 2008
A simpler way to state this is that it is the number of pairs (x,y) where at least one of x and y is the empty set. -Franklin T. Adams-Watters,Oct 28 2011
2^n-1 is the sum of the elements in a Pascal triangle of depth n. - Brian Lewis (bsl04(AT)uark.edu), Feb 26 2008
Sequence generalized: a(n) = (A^n -1)/(A-1), n >= 1, A integer >= 2. This sequence has A=2;A003462has A=3;A002450has A=4;A003463has A=5;A003464has A=6;A023000has A=7;A023001has A=8;A002452has A=9;A002275has A=10;A016123has A=11;A016125has A=12;A091030has A=13;A135519has A=14;A135518has A=15;A131865has A=16;A091045has A=17;A064108has A=20. -Ctibor O. Zizka,Mar 03 2008
a(n) is also a Mersenne numberA001348when n is prime. -Omar E. Pol,Sep 05 2008
With offset 1, = row sums of triangleA144081;and INVERT transform ofA009545starting with offset 1; whereA009545= expansion of sin(x)*exp(x). -Gary W. Adamson,Sep 10 2008
For n > 0, sequence is equal to partial sums ofA000079;a(n) =A000203(A000079(n-1)). -Lekraj Beedassy,May 02 2009
Starting with offset 1 = the Jacobsthal sequence,A001045,(1, 1, 3, 5, 11, 21,...) convolved with (1, 2, 2, 2,...). -Gary W. Adamson,May 23 2009
Numbers n such that n=2*phi(n+1)-1. -Farideh Firoozbakht,Jul 23 2009
a(n) = (a(n-1)+1)-th odd numbers =A005408(a(n-1)) for n >= 1. -Jaroslav Krizek,Sep 11 2009
Partial sums of a(n) for n >= 0 areA000295(n+1). Partial sums of a(n) for n >= 1 areA000295(n+1) andA130103(n+1). a(n) =A006127(n) - (n+1). -Jaroslav Krizek,Oct 16 2009
If n is even a(n) mod 3 = 0. This follows from the congruences 2^(2k) - 1 ~ 2*2*...*2 - 1 ~ 4*4*...*4 - 1 ~ 1*1*...*1 - 1 ~ 0 (mod 3). (Note that 2*2*...*2 has an even number of terms.) -Washington Bomfim,Oct 31 2009
Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=2,(i>1), A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n >= 1, a(n)=det(A). -Milan Janjic,Jan 26 2010
This is the sequence A(0,1;1,2;2) = A(0,1;3,-2;0) of the family of sequences [a,b:c,d:k] considered by G. Detlefs, and treated as A(a,b;c,d;k) in the W. Lang link given below. -Wolfdieter Lang,Oct 18 2010
a(n) = S(n+1,2), a Stirling number of the second kind. See the example below. -Dennis P. Walsh,Mar 29 2011
Entries of row a(n) in Pascal's triangle are all odd, while entries of row a(n)-1 have alternating parities of the form odd, even, odd, even,..., odd.
Define the bar operation as an operation on signed permutations that flips the sign of each entry. Then a(n+1) is the number of signed permutations of length 2n that are equal to the bar of their reverse-complements and avoid the set of patterns {(-2,-1), (-1,+2), (+2,+1)}. (See the Hardt and Troyka reference.) -Justin M. Troyka,Aug 13 2011
This sequence is also the number of proper subsets of a set with n elements. -Mohammad K. Azarian,Oct 27 2011
a(n) is the number k such that the number of iterations of the map k -> (3k +1)/2 == 1 (mod 2) until reaching (3k +1)/2 == 0 (mod 2) equals n. (see the Collatz problem). -Michel Lagneau,Jan 18 2012
For integers a, b, denote by a<+>b the least c >= a such that Hd(a,c) = b (note that, generally speaking, a<+>b differs from b<+>a). Then a(n+1)=a(n)<+>1. Thus this sequence is the Hamming analog of nonnegative integers. -Vladimir Shevelev,Feb 13 2012
Pisano period lengths: 1, 1, 2, 1, 4, 2, 3, 1, 6, 4, 10, 2, 12, 3, 4, 1, 8, 6, 18, 4,... apparentlyA007733.-R. J. Mathar,Aug 10 2012
Start with n. Each n generates a sublist {n-1,n-2,...,1}. Each element of each sublist also generates a sublist. Take the sum of all. E.g., 3->{2,1} and 2->{1}, so a(3)=3+2+1+1=7. -Jon Perry,Sep 02 2012
This is the Lucas U(P=3,Q=2) sequence. -R. J. Mathar,Oct 24 2012
The Mersenne numbers >= 7 are all Brazilian numbers, as repunits in base two. See Proposition 1 & 5.2 in Links: "Les nombres brésiliens". -Bernard Schott,Dec 26 2012
Number of line segments after n-th stage in the H tree. -Omar E. Pol,Feb 16 2013
Row sums of triangle inA162741.-Reinhard Zumkeller,Jul 16 2013
a(n) is the highest power of 2 such that 2^a(n) divides (2^n)!. -Ivan N. Ianakiev,Aug 17 2013
In computer programming, these are the only unsigned numbers such that k&(k+1)=0, where & is the bitwise AND operator and numbers are expressed in binary. -Stanislav Sykora,Nov 29 2013
Minimal number of moves needed to interchange n frogs in the frogs problem (see for example the NRICH 1246 link or the Britton link below). -N. J. A. Sloane,Jan 04 2014
a(n)!== 4 (mod 5); a(n)!== 10 (mod 11); a(n)!== 2, 4, 5, 6 (mod 7). -Carmine Suriano,Apr 06 2014
After 0, antidiagonal sums of the array formed by partial sums of integers (1, 2, 3, 4,...). -Luciano Ancora,Apr 24 2015
a(n+1) equals the number of ternary words of length n avoiding 01,02. -Milan Janjic,Dec 16 2015
With offset 0 and another initial 0, the n-th term of 0, 0, 1, 3, 7, 15,... is the number of commas required in the fully-expanded von Neumann definition of the ordinal number n. For example, 4:= {0, 1, 2, 3}:= {{}, {{}}, {{}, {{}}}, {{}, {{}}, {{}, {{}}}}}, which uses seven commas. Also, for n>0, a(n) is the total number of symbols required in the fully-expanded von Neumann definition of ordinal n - 1, where a single symbol (as usual) is always used to represent the empty set and spaces are ignored. E.g., a(5) = 31, the total such symbols for the ordinal 4. -Rick L. Shepherd,May 07 2016
With the quantum integers defined by [n+1]_q = (q^(n+1) - q^(-n-1)) / (q - q^(-1)), the Mersenne numbers are a(n+1) = q^n [n+1]_q with q = sqrt(2), whereas the signed Jacobsthal numbersA001045are given by q = i * sqrt(2) for i^2 = -1. Cf.A239473.-Tom Copeland,Sep 05 2016
For n>1: numbers n such that n - 1 divides sigma(n + 1). -Juri-Stepan Gerasimov,Oct 08 2016
This is also the second column of the Stirling2 triangleA008277(see alsoA048993). -Wolfdieter Lang,Feb 21 2017
Except for the initial terms, the decimal representation of the x-axis of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 659", "Rule 721" and "Rule 734", based on the 5-celled von Neumann neighborhood initialized with a single on cell. -Robert Price,Mar 14 2017
a(n), n > 1, is the number of maximal subsemigroups of the monoid of order-preserving partial injective mappings on a set with n elements. -James MitchellandWilf A. Wilson,Jul 21 2017
Also the number of independent vertex sets and vertex covers in the complete bipartite graph K_{n-1,n-1}. -Eric W. Weisstein,Sep 21 2017
Sum_{k=0..n} p^k is the determinant of n X n matrix M_(i, j) = binomial(i + j - 1, j)*p + binomial(i+j-1, i), in this case p=2 (empirical observation). -Tony Foster III,May 11 2019
The rational numbers r(n) = a(n+1)/2^(n+1) = a(n+1)/A000079(n+1) appear also as root of the n-th iteration f^{[n]}(c; x) = 2^(n+1)*x - a(n+1)*c of f(c; x) = f^{[0]}(c; x) = 2*x - c as r(n)*c. This entry is motivated by a riddle of Johann Peter Hebel (1760 - 1826): Erstes Rechnungsexempel(Ein merkwürdiges Rechnungs-Exempel) from 1803, with c = 24 and n = 2, leading to the root r(2)*24 = 21 as solution. See the link and reference. For the second problem, also involving the present sequence, see a comment inA130330.-Wolfdieter Lang,Oct 28 2019
a(n) is the sum of the smallest elements of all subsets of {1,2,..,n} that contain n. For example, a(3)=7; the subsets of {1,2,3} that contain 3 are {3}, {1,3}, {2,3}, {1,2,3}, and the sum of smallest elements is 7. -Enrique Navarrete,Aug 21 2020
a(n-1) is the number of nonempty subsets of {1,2,..,n} which don't have an element that is the size of the set. For example, for n = 4, a(3) = 7 and the subsets are {2}, {3}, {4}, {1,3}, {1,4}, {3,4}, {1,2,4}. -Enrique Navarrete,Nov 21 2020
FromEric W. Weisstein,Sep 04 2021: (Start)
Also the number of dominating sets in the complete graph K_n.
Also the number of minimum dominating sets in the n-helm graph for n >= 3. (End)
Conjecture: except for a(2)=3, numbers m such that 2^(m+1) - 2^j - 2^k - 1 is composite for all 0 <= j < k <= m. -Chai Wah Wu,Sep 08 2021
a(n) is the number of three-in-a-rows passing through a corner cell in n-dimensional tic-tac-toe. -Ben Orlin,Mar 15 2022
FromVladimir Pletser,Jan 27 2023: (Start)
a(n) == 1 (mod 30) for n == 1 (mod 4);
a(n) == 7 (mod 120) for n == 3 (mod 4);
(a(n) - 1)/30 = (a(n+2) - 7)/120 for n odd;
(a(n) - 1)/30 = (a(n+2) - 7)/120 =A131865(m) for n == 1 (mod 4) and m >= 0 withA131865(0) = 0. (End)
a(n) is the number of n-digit numbers whose smallest decimal digit is 8. -Stefano Spezia,Nov 15 2023
Also, number of nodes in a perfect binary tree of height n-1, or: number of squares (or triangles) after the n-th step of the construction of a Pythagorean tree: Start with a segment. At each step, construct squares having the most recent segment(s) as base, and isosceles right triangles having the opposite side of the squares as hypotenuse ( "on top" of each square). The legs of these triangles will serve as the segments which are the bases of the squares in the next step. -M. F. Hasler,Mar 11 2024
a(n) is the length of the longest path in the n-dimensional hypercube. -Christian Barrientos,Apr 13 2024
a(n) is the diameter of the n-Hanoi graph. Equivalently, a(n) is the largest minimum number of moves between any two states of the Towers of Hanoi problem (aka problem of Benares Temple described above). -Allan Bickle,Aug 09 2024
REFERENCES
P. Bachmann, Niedere Zahlentheorie (1902, 1910), reprinted Chelsea, NY, 1968, vol. 2, p. 75.
Ralph P. Grimaldi, Discrete and Combinatorial Mathematics: An Applied Introduction, Fifth Edition, Addison-Wesley, 2004, p. 134.
Johann Peter Hebel, Gesammelte Werke in sechs Bänden, Herausgeber: Jan Knopf, Franz Littmann und Hansgeorg Schmidt-Bergmann unter Mitarbeit von Ester Stern, Wallstein Verlag, 2019. Band 3, S. 20-21, Loesung, S. 36-37. See also the link below.
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).
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers, "Tower of Hanoi", Penguin Books, 1987, pp. 112-113.
LINKS
Franklin T. Adams-Watters,Table of n, a(n) for n = 0..1000
Omran Ahmadi and Robert Granger,An efficient deterministic test for Kloosterman sum zeros,Math. Comp. 83 (2014), 347-363, arXiv:1104.3882[math.NT], 2011-2012. See 1st and 2nd column of Table 1 p. 9.
Feryal Alayont and Evan Henning,Edge Covers of Caterpillars, Cycles with Pendants, and Spider Graphs,J. Int. Seq. (2023) Vol. 26, Art. 23.9.4.
Anonymous,The Tower of Hanoi
M. Baake, F. Gahler and U. Grimm,Examples of substitution systems and their factors,arXiv:1211.5466 [math.DS], 2012-2013.
Michael Baake, Franz Gähler, and Uwe Grimm,Examples of Substitution Systems and Their Factors,Journal of Integer Sequences, Vol. 16 (2013), #13.2.14.
J.-L. Baril,Classical sequences revisited with permutations avoiding dotted pattern,Electronic Journal of Combinatorics, 18 (2011), #P178.
Paul Barry,A Catalan Transform and Related Transformations on Integer Sequences,Journal of Integer Sequences, Vol. 8 (2005), Article 05.4.5.
Jonathan Beagley and Lara Pudwell,Colorful Tilings and Permutations,Journal of Integer Sequences, Vol. 24 (2021), Article 21.10.4.
J. Bernheiden,Mersennesche Zahlen,(Text in German) [Wayback Machine cached version].
Michael Boardman,The Egg-Drop Numbers,Mathematics Magazine, 77 (2004), 368-372.
R. P. Brent and H. J. J. te Riele,Factorizations of a^n +- 1, 13 <= a < 100,CWI Report 9212, 1992 [Wayback Machine cached version].
R. P. Brent, P. L. Montgomery and H. J. J. te Riele,Factorizations of a^n +- 1, 13 <= a < 100: Update 2
R. P. Brent, P. L. Montgomery and H. J. J. te Riele,Factorizations Of Cunningham Numbers With Bases 13 To 99. Millennium Edition[BROKEN LINK]
R. P. Brent, P. L. Montgomery and H. J. J. te Riele,Factorizations of Cunningham numbers with bases 13 to 99: Millennium edition
R. P. Brent and H. J. J. te Riele,Factorizations of a^n +- 1, 13 <= a < 100
John Brillhart et al.,Cunningham Project[Factorizations of b^n +- 1, b = 2, 3, 5, 6, 7, 10, 11, 12 up to high powers] [Subscription required].
Jill Britton,The Tower of Hanoi[Video file, Wayback Machine cached version].
Jill Britton,The Frog Puzzle[Wayback Machine cached version].
C. K. Caldwell, The Prime Glossary,Mersenne number
Naiomi T. Cameron and Asamoah Nkwanta,On Some (Pseudo) Involutions in the Riordan Group,Journal of Integer Sequences, Vol. 8 (2005), Article 05.3.7.
P. J. Cameron,Sequences realized by oligomorphic permutation groups,J. Integ. Seqs. Vol. 3 (2000), #00.1.5.
P. Catarino, H. Campos, and P. Vasco,On the Mersenne sequence,Annales Mathematicae et Informaticae, 46 (2016) pp. 37-53.
Robert George Cowell,A unifying framework for the modelling and analysis of STR DNA samples arising in forensic casework,arXiv:1802.09863 [stat.AP], 2018.
F. Javier de Vega,An extension of Furstenberg's theorem of the infinitude of primes,arXiv:2003.13378 [math.NT], 2020.
W. M. B. Dukes,On the number of matroids on a finite set,arXiv:math/0411557 [math.CO], 2004.
James East, Jitender Kumar, James D. Mitchell, and Wilf A. Wilson,Maximal subsemigroups of finite transformation and partition monoids,arXiv:1706.04967 [math.GR], 2017. [James MitchellandWilf A. Wilson,Jul 21 2017]
W. Edgington,Mersenne Page[BROKEN LINK]
David Eppstein,Making Change in 2048,arXiv:1804.07396 [cs.DM], 2018.
T. Eveilleau,Animated solution to the Tower of Hanoi problem
G. Everest et al.,Primes generated by recurrence sequences,Amer. Math. Monthly, 114 (No. 5, 2007), 417-431.
G. Everest, S. Stevens, D. Tamsett and T. Ward,Primitive divisors of quadratic polynomial sequences,arXiv:math/0412079 [math.NT], 2004-2006.
G. Everest, A. J. van der Poorten, Y. Puri and T. Ward,Integer Sequences and Periodic Points,Journal of Integer Sequences, Vol. 5 (2002), Article 02.2.3.
Bakir Farhi,Summation of Certain Infinite Lucas-Related Series,J. Int. Seq., Vol. 22 (2019), Article 19.1.6.
Emmanuel Ferrand,Deformations of the Taylor Formula,Journal of Integer Sequences, Vol. 10 (2007), Article 07.1.7.
Robert Frontczak and Taras Goy,Mersenne-Horadam identities using generating functions,Carpathian Mathematical Publications, Vol. 12, no. 1, (2020), 34-45.
Robert Granger,On the Enumeration of Irreducible Polynomials over GF(q) with Prescribed Coefficients,arXiv:1610.06878 [math.AG], 2016. See 1st and 2nd column of Table 1 p. 13.
Taras Goy,On new identities for Mersenne numbers,Applied Mathematics E-Notes, 18 (2018), 100-105.
R. K. Guy,Letter to N. J. A. Sloane
A. Hardt and J. M. Troyka,Restricted symmetric signed permutations,Pure Mathematics and Applications, Vol. 23 (No. 3, 2012), pp. 179--217.
A. Hardt and J. M. Troyka,Slides(associated with the Hardt and Troyka reference above).
Johann Peter Hebel,Erstes Rechnungsexempel,1803; Solution:Auflösung des ersten Rechnungsexempels und ein zweites,1804.
A. M. Hinz, S. Klavžar, U. Milutinović, and C. Petr,The Tower of Hanoi - Myths and Maths,Birkhäuser 2013. See page 11.Book's website
A. Hinz, S. Klavzar, and S. Zemljic,A survey and classification of Sierpinski-type graphs,Discrete Applied Mathematics 217 3 (2017), 565-600.
Andreas M. Hinz and Paul K. Stockmeyer,Precious Metal Sequences and Sierpinski-Type Graphs,J. Integer Seq., Vol 25 (2022), Article 22.4.8.
Jiří Klaška,Jakóbczyk's Hypothesis on Mersenne Numbers and Generalizations of Skula's Theorem,J. Int. Seq., Vol. 26 (2023), Article 23.3.8.
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.
Wolfdieter Lang,Notes on certain inhomogeneous three term recurrences.
J. Loy,The Tower of Hanoi
Edouard Lucas,The Theory of Simply Periodic Numerical Functions,Fibonacci Association, 1969. English translation of article "Théorie des Fonctions Numériques Simplement Périodiques, I", Amer. J. Math., 1 (1878), 184-240.
Mathforum,Tower of Hanoi
Mathforum, Problem of the Week,The Tower of Hanoi Puzzle
Donatella Merlini and Massimo Nocentini,Algebraic Generating Functions for Languages Avoiding Riordan Patterns,Journal of Integer Sequences, Vol. 21 (2018), Article 18.1.3.
R. Mestrovic,Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC--2012) and another new proof,arXiv preprint arXiv:1202.3670 [math.HO], 2012. - FromN. J. A. Sloane,Jun 13 2012
N. Moreira and R. Reis,On the Density of Languages Representing Finite Set Partitions,Journal of Integer Sequences, Vol. 8 (2005), Article 05.2.8.
NRICH 1246,Frogs
Ahmet Öteleş,Bipartite Graphs Associated with Pell, Mersenne and Perrin Numbers,An. Şt. Univ. Ovidius Constantą, (2019) Vol. 27, Issue 2, 109-120.
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Simon Plouffe,1031 Generating Functions,Appendix to Thesis, Montreal, 1992
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R. R. Snapp,The Tower of Hanoi
Amelia Carolina Sparavigna,On the generalized sums of Mersenne, Fermat, Cullen and Woodall Numbers,Politecnico di Torino (Italy, 2019).
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Thesaurus.maths.org,Mersenne Number
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Eric Weisstein's World of Mathematics,Complete Bipartite Graph
Eric Weisstein's World of Mathematics,Complete
Eric Weisstein's World of Mathematics,Dominating Set
Eric Weisstein's World of Mathematics,Helm Graph
Eric Weisstein's World of Mathematics,Independent Vertex Set
Eric Weisstein's World of Mathematics,Mersenne Number
Eric Weisstein's World of Mathematics,Minimum Dominating Set
Eric Weisstein's World of Mathematics,Vertex Cover
K. K. Wong,Tower Of Hanoi:Online Game
K. Zsigmondy,Zur Theorie der Potenzreste,Monatsh. Math., 3 (1892), 265-284.
Index entries for linear recurrences with constant coefficients,signature (3,-2).
FORMULA
G.f.: x/((1-2*x)*(1-x)).
E.g.f.: exp(2*x) - exp(x).
E.g.f. if offset 1: ((exp(x)-1)^2)/2.
a(n) = Sum_{k=0..n-1} 2^k. -Paul Barry,May 26 2003
a(n) = a(n-1) + 2*a(n-2) + 2, a(0)=0, a(1)=1. -Paul Barry,Jun 06 2003
Let b(n) = (-1)^(n-1)*a(n). Then b(n) = Sum_{i=1..n} i!*i*Stirling2(n,i)*(-1)^(i-1). E.g.f. of b(n): (exp(x)-1)/exp(2x). - Mario Catalani (mario.catalani(AT)unito.it), Dec 19 2003
a(n+1) = 2*a(n) + 1, a(0) = 0.
a(n) = Sum_{k=1..n} binomial(n, k).
a(n) = n + Sum_{i=0..n-1} a(i); a(0) = 0. -Rick L. Shepherd,Aug 04 2004
a(n+1) = (n+1)*Sum_{k=0..n} binomial(n, k)/(k+1). -Paul Barry,Aug 06 2004
a(n+1) = Sum_{k=0..n} binomial(n+1, k+1). -Paul Barry,Aug 23 2004
Inverse binomial transform ofA001047.Also U sequence of Lucas sequence L(3, 2). -Ross La Haye,Feb 07 2005
a(n) =A119258(n,n-1) for n > 0. -Reinhard Zumkeller,May 11 2006
a(n) = 3*a(n-1) - 2*a(n-2); a(0)=0, a(1)=1. -Lekraj Beedassy,Jun 07 2006
Stirling_2(n-k,2) starting from n=k+1. -Artur Jasinski,Nov 18 2006
a(n) =A125118(n,1) for n > 0. -Reinhard Zumkeller,Nov 21 2006
a(n) = StirlingS2(n+1,2). -Ross La Haye,Jan 10 2008
For n > 0:A179857(a(n)) =A024036(n) andA179857(m) <A024036(n) for m < a(n). -Reinhard Zumkeller,Jul 31 2010
FromEnrique Pérez Herrero,Aug 21 2010: (Start)
a(n) = J_n(2), where J_n is the n-th Jordan Totient function: (A007434,is J_2).
a(n) = Sum_{d|2} d^n*mu(2/d). (End)
A036987(a(n)) = 1. -Reinhard Zumkeller,Mar 06 2012
a(n) =A007283(n)/3 - 1. -Martin Ettl,Nov 11 2012
a(n) = det(|s(i+2,j+1)|, 1 <= i,j <= n-1), where s(n,k) are Stirling numbers of the first kind. -Mircea Merca,Apr 06 2013
G.f.: Q(0), 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 22 2013
E.g.f.: Q(0), where Q(k) = 1 - 1/(2^k - 2*x*4^k/(2*x*2^k - (k+1)/Q(k+1))); (continued fraction).
G.f.: Q(0), where Q(k) = 1 - 1/(2^k - 2*x*4^k/(2*x*2^k - 1/Q(k+1))); (continued fraction). -Sergei N. Gladkovskii,May 23 2013
a(n) =A000203(2^(n-1)), n >= 1. -Ivan N. Ianakiev,Aug 17 2013
a(n) = Sum_{t_1+2*t_2+...+n*t_n=n} n*multinomial(t_1+t_2 +...+t_n,t_1,t_2,...,t_n)/(t_1+t_2 +...+t_n). -Mircea Merca,Dec 06 2013
a(0) = 0; a(n) = a(n-1) + 2^(n-1) for n >= 1. -Fred Daniel Kline,Feb 09 2014
FromIlya Gutkovskiy,Aug 07 2016: (Start)
Binomial transform ofA057427.
Sum_{n>=0} a(n)/n! =A090142.(End)
a(n) =A000918(n) + 1. -Miquel Cerda,Aug 09 2016
Convolution of binomial coefficient C(n,a(k)) with itself is C(n,a(k+1)) for all k >= 3. -Anton Zakharov,Sep 05 2016
a(n) =A279396(n+2,2). -Wolfdieter Lang,Jan 10 2017
a(n) = n + Sum_{j=1..n-1} (n-j)*2^(j-1). See a Jun 14 2017 formula forA000918(n+1) with an interpretation. -Wolfdieter Lang,Jun 14 2017
a(n) = Sum_{k=0..n-1} Sum_{i=0..n-1} C(k,i). -Wesley Ivan Hurt,Sep 21 2017
a(n+m) = a(n)*a(m) + a(n) + a(m). -Yuchun Ji,Jul 27 2018
a(n+m) = a(n+1)*a(m) - 2*a(n)*a(m-1). -Taras Goy,Dec 23 2018
a(n+1) is the determinant of n X n matrix M_(i, j) = binomial(i + j - 1, j)*2 + binomial(i+j-1, i) (empirical observation). -Tony Foster III,May 11 2019
EXAMPLE
For n=3, a(3)=S(4,2)=7, a Stirling number of the second kind, since there are 7 ways to partition {a,b,c,d} into 2 nonempty subsets, namely,
{a}U{b,c,d}, {b}U{a,c,d}, {c}U{a,b,d}, {d}U{a,b,c}, {a,b}U{c,d}, {a,c}U{b,d}, and {a,d}U{b,c}. -Dennis P. Walsh,Mar 29 2011
FromJustin M. Troyka,Aug 13 2011: (Start)
Since a(3) = 7, there are 7 signed permutations of 4 that are equal to the bar of their reverse-complements and avoid {(-2,-1), (-1,+2), (+2,+1)}. These are:
(+1,+2,-3,-4),
(+1,+3,-2,-4),
(+1,-3,+2,-4),
(+2,+4,-1,-3),
(+3,+4,-1,-2),
(-3,+1,-4,+2),
(-3,-4,+1,+2). (End)
G.f. = x + 3*x^2 + 7*x^3 + 15*x^4 + 31*x^5 + 63*x^6 + 127*x^7 +...
For the Towers of Hanoi problem with 2 disks, the moves are as follows, so a(2) = 3.
12|_|_ -> 2|1|_ -> _|1|2 -> _|_|12 -Allan Bickle,Aug 07 2024
MAPLE
A000225:= n->2^n-1; [ seq(2^n-1, n=0..50) ];
A000225:=1/(2*z-1)/(z-1); #Simon Plouffein his 1992 dissertation, sequence starting at a(1)
MATHEMATICA
a[n_]:= 2^n - 1; Table[a[n], {n, 0, 30}] (*Stefan Steinerberger,Mar 30 2006 *)
Array[2^# - 1 &, 50, 0] (* Joseph Biberstine (jrbibers(AT)indiana.edu), Dec 26 2006 *)
NestList[2 # + 1 &, 0, 32] (*Robert G. Wilson v,Feb 28 2011 *)
2^Range[0, 20] - 1 (*Eric W. Weisstein,Jul 17 2017 *)
LinearRecurrence[{3, -2}, {1, 3}, 20] (*Eric W. Weisstein,Sep 21 2017 *)
CoefficientList[Series[1/(1 - 3 x + 2 x^2), {x, 0, 20}], x] (*Eric W. Weisstein,Sep 21 2017 *)
PROG
(PARI)A000225(n) = 2^n-1 \\Michael B. Porter,Oct 27 2009
(Haskell)
a000225 = (subtract 1). (2 ^)
a000225_list = iterate ((+ 1). (* 2)) 0
--Reinhard Zumkeller,Mar 20 2012
(PARI) concat(0, Vec(x/((1-2*x)*(1-x)) + O(x^100))) \\Altug Alkan,Oct 28 2015
(SageMath)
def isMersenne(n): return n == sum([(1 - b) << s for (s, b) in enumerate((n+1).bits())]) #Peter Luschny,Sep 01 2019
(Python)
defA000225(n): return (1<<n)-1 #Chai Wah Wu,Jul 06 2022
CROSSREFS
Cf.A000043(Mersenne exponents).
Cf.A000668(Mersenne primes).
Cf.A001348(Mersenne numbers with n prime).
Subsequence ofA132781.
KEYWORD
nonn,easy,core,nice
AUTHOR
EXTENSIONS
Name partially edited byEric W. Weisstein,Sep 04 2021
STATUS
approved