

A000079


Powers of 2: a(n) = 2^n.
(Formerly M1129 N0432)


3137



1, 2, 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
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OFFSET

0,2


COMMENTS

2^0 = 1 is the only odd power of 2.
Number of subsets of an nset.
There are 2^(n1) compositions (ordered partitions) of n (see for example Riordan). This is the unlabeled analog of the preferential labelings sequence A000670.
This is also the number of weakly unimodal permutations of 1..n + 1, that is, permutations with exactly one local maximum. E.g., a(4) = 16: 12345, 12354, 12453, 12543, 13452, 13542, 14532 and 15432 and their reversals.  Jon Perry, Jul 27 2003 [Proof: see next line! See also A087783.]
Proof: n must appear somewhere and there are 2^(n1) possible choices for the subset that precedes it. These must appear in increasing order and the rest must follow n in decreasing order. QED.  N. J. A. Sloane, Oct 26 2003
a(n+1) is the smallest number that is not the sum of any number of (distinct) earlier terms.
Same as Pisot sequences E(1, 2), L(1, 2), P(1, 2), T(1, 2). See A008776 for definitions of Pisot sequences.
With initial 1 omitted, same as Pisot sequences E(2, 4), L(2, 4), P(2, 4), T(2, 4).  David W. Wilson
Not the sum of two or more consecutive numbers.  Lekraj Beedassy, May 14 2004
Least deficient or nearperfect numbers (i.e., n such that sigma(n) = A000203(n) = 2n  1).  Lekraj Beedassy, Jun 03 2004. [Comment from Max Alekseyev, Jan 26 2005: All the powers of 2 are least deficient numbers but it is not known if there exists a least deficient number that is not a power of 2.]
Almostperfect numbers referred to as least deficient or slightly defective (Singh 1997) numbers. Does "nearperfect numbers" refer to both almostperfect numbers (sigma(n) = 2n  1) and quasiperfect numbers (sigma(n) = 2n + 1)? There are no known quasiperfect or least abundant or slightly excessive (Singh 1997) numbers.
The sum of the numbers in the nth row of Pascal's triangle; the sum of the coefficients of x in the expansion of (x+1)^n.
The Collatz conjecture (the hailstone sequence will eventually reach the number 1, regardless of which positive integer is chosen initially) may be restated as (the hailstone sequence will eventually reach a power of 2, regardless of which positive integer is chosen initially).
The only hailstone sequence which doesn't rebound (except "on the ground").  Alexandre Wajnberg, Jan 29 2005
With p(n) as the number of integer partitions of n, p(i) is the number of parts of the ith partition of n, d(i) is the number of different parts of the ith partition of n, m(i,j) is the multiplicity of the jth part of the ith partition of n, one has: a(n) = Sum_{i = 1..p(n)} (p(i)! / (Product_{j=1..d(i)} m(i,j)!)).  Thomas Wieder, May 18 2005
The number of binary relations on an nelement set that are both symmetric and antisymmetric. Also the number of binary relations on an nelement set that are symmetric, antisymmetric and transitive.
a(n) is the largest number with shortest addition chain involving n additions.  David W. Wilson, Apr 23 2006
Beginning with a(1) = 0, numbers not equal to the sum of previous distinct natural numbers.  Giovanni Teofilatto, Aug 06 2006
For n >= 1, a(n) is equal to the number of functions f:{1, 2, ..., n} > {1, 2} such that for a fixed x in {1, 2, ..., n} and a fixed y in {1, 2} we have f(x) != y.  Aleksandar M. Janjic and Milan Janjic, Mar 27 2007
Let P(A) be the power set of an nelement set A. Then a(n) is the number of pairs of elements {x,y} of P(A) for which x = y.  Ross La Haye, Jan 09 2008
a(n) is the number of different ways to run up a staircase with n steps, taking steps of sizes 1, 2, 3, ... and r (r <= n), where the order IS important and there is no restriction on the number or the size of each step taken.  Mohammad K. Azarian, May 21 2008
a(n) is the number of permutations on [n+1] such that every initial segment is an interval of integers. Example: a(3) counts 1234, 2134, 2314, 2341, 3214, 3241, 3421, 4321. The map "p > ascents of p" is a bijection from these permutations to subsets of [n]. An ascent of a permutation p is a position i such that p(i) < p(i+1). The permutations shown map to 123, 23, 13, 12, 3, 2, 1 and the empty set respectively.  David Callan, Jul 25 2008
2^(n1) is the largest number having n divisors (in the sense of A077569); A005179(n) is the smallest.  T. D. Noe, Sep 02 2008
a(n) appears to match the number of divisors of the modified primorials (excluding 2, 3 and 5). Very limited range examined, PARI example shown.  Bill McEachen, Oct 29 2008
Successive k such that phi(k)/k = 1/2, where phi is Euler's totient function.  Artur Jasinski, Nov 07 2008
This is also the (L)sieve transform of {2, 4, 6, 8, ..., 2n, ...} = A005843. (See A152009 for the definition of the (L)sieve transform.)  John W. Layman, Jan 23 2009
For n >= 0, a(n) is the number of leaves in a complete binary tree of height n. For n > 0, a(n) is the number of nodes in an ncube.  K.V.Iyer, May 04 2009
Permutations of n+1 elements where no element is more than one position right of its original place. For example, there are 4 such permutations of three elements: 123, 132, 213, and 312. The 8 such permutations of four elements are 1234, 1243, 1324, 1423, 2134, 2143, 3124, and 4123.  Joerg Arndt, Jun 24 2009
a(n) written in base 2: 1,10,100,1000,10000,..., i.e., (n+1) times 1, n times 0 (A011557(n)).  Jaroslav Krizek, Aug 02 2009
These are the 2smooth numbers, positive integers with no prime factors greater than 2.  Michael B. Porter, Oct 04 2009
a(n) is the largest number m such that the number of steps of iterations of {r  (largest divisor d < r)} needed to reach 1 starting at r = m is equal to n. Example (a(5) = 32): 32  16 = 16; 16  8 = 8; 8  4 = 4; 4  2 = 2; 2  1 = 1; number 32 has 5 steps and is the largest such number. See A105017, A064097, A175125.  Jaroslav Krizek, Feb 15 2010
The powersof2 triangle T(n, k), n >= 0 and 0 <= k <= n, begins with: {1}; {2, 4}; {8, 16, 32}; {64, 128, 256, 512}; ... . The first left hand diagonal T(n, 0) = A006125(n + 1), the first right hand diagonal T(n, n) = A036442(n + 1) and the center diagonal T(2*n, n) = A053765(n + 1). Some triangle sums, see A180662, are: Row1(n) = A122743(n), Row2(n) = A181174(n), Fi1(n) = A181175(n), Fi2(2*n) = A181175(2*n) and Fi2(2*n + 1) = 2*A181175(2*n + 1).  Johannes W. Meijer, Oct 10 2010
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>=1, a(n) equals the number of 2colored compositions of n such that no adjacent parts have the same color.  Milan Janjic, Nov 17 2011
Equals A001405 convolved with its rightshifted variant: (1 + 2x + 4x^2 + ...) = (1 + x + 2x^2 + 3x^3 + 6x^4 + 10x^5 + ...) * (1 + x + x^2 + 2x^3 + 3x^4 + 6x^5 + ...).  Gary W. Adamson, Nov 23 2011
The number of oddsized subsets of an n+1set. For example, there are 2^3 oddsized subsets of {1, 2, 3, 4}, namely {1}, {2}, {3}, {4}, {1, 2, 3}, {1, 2, 4}, {1, 3, 4}, and {2, 3, 4}. Also, note that 2^n = Sum_{k=1..floor((n+1)/2)} C(n+1, 2k1).  Dennis P. Walsh, Dec 15 2011
a(n) is the number of 1's in any row of Pascal's triangle (mod 2) whose row number has exactly n 1's in its binary expansion (see A007318 and A047999). (The result of putting together A001316 and A000120.)  Marcus Jaiclin, Jan 31 2012
For n>=1 apparently the number of distinct finite languages over a unary alphabet, whose minimum regular expression has alphabetic width n (verified up to n=17), see the Gruber/Lee/Shallit link.  Hermann Gruber, May 09 2012
This is the lexicographically earliest sequence which contains no arithmetic progression of length 3.  Daniel E. Frohardt, Apr 03 2013
a(n2) is the number of bipartitions of {1..n} (i.e., set partitions into two parts) such that 1 and 2 are not in the same subset.  Jon Perry, May 19 2013
Numbers n such that the nth cyclotomic polynomial has a root mod 2; numbers n such that the nth cyclotomic polynomial has an even number of odd coefficients.  Eric M. Schmidt, Jul 31 2013
More is known now about nonpowerof2 "Almost Perfect Numbers" as described in Dagal.  Jonathan Vos Post, Sep 01 2013
Number of symmetric Ferrers diagrams that fit into an n X n box.  Graham H. Hawkes, Oct 18 2013
a(1), ..., a(floor(n/2)) are all values of permanent on set of square (0,1)matrices of order n>=2 with row and column sums 2.  Vladimir Shevelev, Nov 26 2013
Numbers whose base2 expansion has exactly one bit set to 1, and thus has base2 sum of digits equal to one.  Stanislav Sykora, Nov 29 2013
a(n) is the largest number k such that (k^n2)/(k2) is an integer (for n > 1); (k^a(n)+1)/(k+1) is never an integer (for k > 1 and n > 0).  Derek Orr, May 22 2014
The minisequence b(n) = least number k > 0 such that 2^k ends in n identical digits is given by {1, 18, 39}. The repeating digits are {2, 4, 8} respectively. Note that these are consecutive powers of 2 (2^1, 2^2, 2^3), and these are the only powers of 2 (2^k, k > 0) that are only one digit. Further, this sequence is finite. The number of ndigit endings for a power of 2 with n or more digits id 4*5^(n1). Thus, for b(4) to exist, one only needs to check exponents up to 4*5^3 = 500. Since b(4) does not exist, it is clear that no other number will exist.  Derek Orr, Jun 14 2014
The least number k > 0 such that 2^k ends in n consecutive decreasing digits is a 3number sequence given by {1, 5, 25}. The consecutive decreasing digits are {2, 32, 432}. There are 100 different 3digit endings for 2^k. There are no kvalues such that 2^k ends in '987', '876', '765', '654', '543', '321', or '210'. The kvalues for which 2^k ends in '432' are given by 25 mod 100. For k = 25 + 100*x, the digit immediately before the run of '432' is {4, 6, 8, 0, 2, 4, 6, 8, 0, 2, ...} for x = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ...}, respectively. Thus, we see the digit before '432' will never be a 5. So, this sequence is complete.  Derek Orr, Jul 03 2014
a(n) is the number of permutations of length n avoiding both 231 and 321 in the classical sense which are breadthfirst search reading words of increasing unarybinary trees. For more details, see the entry for permutations avoiding 231 at A245898.  Manda Riehl, Aug 05 2014
This is a B_2 sequence: for i < j, differences a(j)  a(i) are all distinct. Here 2*a(n) < a(n+1) + 1, so a(n)  a(0) < a(n+1)  a(n).  Thomas Ordowski, Sep 23 2014
a(n) counts nwalks (closed) on the graph G(1vertex; 1loop, 1loop).  David Neil McGrath, Dec 11 2014
a(n1) counts walks (closed) on the graph G(1vertex; 1loop, 2loop, 3loop, 4loop, ...).  David Neil McGrath, Jan 01 2015
b(0) = 4; b(n+1) is the smallest number not in the sequence such that b(n+1)  Prod_{i=0..n} b(i) divides b(n+1)  Sum_{i=0..n} b(i). Then b(n) = a(n) for n > 2.  Derek Orr, Jan 15 2015
a(n) counts the permutations of length n+2 whose first element is 2 such that the permutation has exactly one descent.  Ran Pan, Apr 17 2015
a(0)a(30) appear, with a(26)a(30) in error, in tablet M 08613 (see CDLI link) from the Old Babylonian period (c. 19001600 BC).  Charles R Greathouse IV, Sep 03 2015
Number of monotone maps f : [0..n] > [0..n] which are orderincreasing (i <= f(i)) and idempotent (f(f(i)) = f(i)). In other words, monads on the nth ordinal (seen as a posetal category). Any monad f determines a subset of [0..n] that contains n, by considering its set of monad algebras = fixed points { i  f(i) = i }. Conversely, any subset S of [0..n] containing n determines a monad on [0..n], by the function i > min { j  i <= j, j in S }.  Noam Zeilberger, Dec 11 2016
Consider n points lying on a circle. Then for n>=2 a(n2) gives the number of ways to connect two adjacent points with nonintersecting chords.  Anton Zakharov, Dec 31 2016
Satisfies Benford's law [Diaconis, 1977; BergerHill, 2017]  N. J. A. Sloane, Feb 07 2017
Also the number of independent vertex sets and vertex covers in the nempty graph.  Eric W. Weisstein, Sep 21 2017
Also the number of maximum cliques in the nhalved cube graph for n > 4.  Eric W. Weisstein, Dec 04 2017
Number of pairs of compositions of n corresponding to a seaweed algebra of index n1.  Nick Mayers, Jun 25 2018
The multiplicative group of integers modulo a(n) is cyclic if and only if n = 0, 1, 2. For n >= 3, it is a product of two cyclic groups.  Jianing Song, Jun 27 2018
k^n is the determinant of n X n matrix M_(i, j) = binomial(k + i + j  2, j)  binomial(i+j2, j), in this case k=2.  Tony Foster III, May 12 2019
a(n1) is the number of subsets of {1,2,...,n} which have an element that is the size of the set. For example, for n = 4, a(3) = 8 and the subsets are {1}, {1,2}, {2,3}, {2,4}, {1,2,3}, {1,3,4}, {2,3,4}, {1,2,3,4}.  Enrique Navarrete, Nov 21 2020
a(n) is the number of selfinverse (n+1)order permutations with 231avoiding. E.g., a(3) = 8: [1234, 1243, 1324, 1432, 2134, 2143, 3214, 4321].  Yuchun Ji, Feb 26 2021
For any fixed k > 0, a(n) is the number of ways to tile a strip of length n+1 with tiles of length 1, 2, ... k, where the tile of length k can be black or white, with the restriction that the first tile cannot be black.  Greg Dresden and Bora Bursalı, Aug 31 2023


REFERENCES

Milton Abramowitz and Irene A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 1016.
Mohammad K. Azarian, A Generalization of the Climbing Stairs Problem, Mathematics and Computer Education Journal, Vol. 31, No. 1, pp. 2428, Winter 1997.
Paul J. Nahin, An Imaginary Tale: The Story of sqrt(1), Princeton University Press, Princeton, NJ. 1998, pp. 6970.
J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 124.
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).
V. E. Tarakanov, Combinatorial problems on binary matrices, Combin. Analysis, MSU, 5 (1980), 415. (Russian)
S. Wolfram, A New Kind of Science, Wolfram Media, 2002; p. 55.


LINKS

Milton Abramowitz and Irene A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
Anicius Manlius Severinus Boethius, De arithmetica, Book 1, section 9.
Eric Weisstein's World of Mathematics, Abundance
Eric Weisstein's World of Mathematics, Erf
Eric Weisstein's World of Mathematics, Hypercube
Eric Weisstein's World of Mathematics, Subset


FORMULA

a(n) = 2^n.
a(0) = 1; a(n) = 2*a(n1).
G.f.: 1/(1  2*x).
E.g.f.: exp(2*x).
a(n)= Sum_{k = 0..n} binomial(n, k).
a(n) is the number of occurrences of n in A000523. a(n) = A001045(n) + A001045(n+1). a(n) = 1 + Sum_{k = 0..(n  1)} a(k). The Hankel transform of this sequence gives A000007 = [1, 0, 0, 0, 0, 0, ...].  Philippe Deléham, Feb 25 2004
a(n + 1) = a(n) XOR 3*a(n) where XOR is the binary exclusive OR operator.  Philippe Deléham, Jun 19 2005
a(n) = StirlingS2(n + 1, 2) + 1.  Ross La Haye, Jan 09 2008
a(n+2) = 6a(n+1)  8a(n), n = 1, 2, 3, ... with a(1) = 1, a(2) = 2.  Yosu Yurramendi, Aug 06 2008
a(n) = ka(n1) + (4  2k)a(n2) for any integer k and n > 1, with a(0) = 1, a(1) = 2.  Jaume Oliver Lafont, Dec 05 2008
a(n) = Sum_{l_1 = 0..n + 1} Sum_{l_2 = 0..n}...Sum_{l_i = 0..n  i}...Sum_{l_n = 0..1} delta(l_1, l_2, ..., l_i, ..., l_n) where delta(l_1, l_2, ..., l_i, ..., l_n) = 0 if any l_i <= l_(i+1) and l_(i+1) != 0 and delta(l_1, l_2, ..., l_i, ..., l_n) = 1 otherwise.  Thomas Wieder, Feb 25 2009
If p[i] = i  1 and if A is the Hessenberg matrix of order n defined by: A[i, j] = p[j  i + 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 02 2010
If p[i] = Fibonacci(i2) and if A is the Hessenberg matrix of order n defined by: A[i, j] = p[j  i + 1], (i <= j), A[i, j] = 1, (i = j + 1), and A[i, j] = 0 otherwise. Then, for n >= 2, a(n2) = det A.  Milan Janjic, May 08 2010
The sum of reciprocals, 1/1 + 1/2 + 1/4 + 1/8 + ... + 1/(2^n) + ... = 2.  Mohammad K. Azarian, Dec 29 2010
a(n) = Hypergeometric([n], [], 1).  Peter Luschny, Nov 01 2011
G.f.: A(x) = B(x)/x, B(x) satisfies B(B(x)) = x/(1  x)^2.  Vladimir Kruchinin, Nov 10 2011
2^n = Sum_{k = 1..floor((n+1)/2)} C(n+1, 2k1).  Dennis P. Walsh, Dec 15 2011
Sum_{n >= 1} mobius(n)/a(n) = 0.1020113348178103647430363939318...  R. J. Mathar, Aug 12 2012
E.g.f.: 1 + 2*x/(U(0)  x) where U(k) = 6*k + 1 + x^2/(6*k+3 + x^2/(6*k + 5 + x^2/U(k+1) )); (continued fraction, 3step).  Sergei N. Gladkovskii, Dec 04 2012
a(n) = det(s(i+2,j), 1 <= i,j <= n), where s(n,k) are Stirling numbers of the first kind.  Mircea Merca, Apr 04 2013
a(n) = det(ps(i+1,j), 1 <= i,j <= n), where ps(n,k) are LegendreStirling numbers of the first kind (A129467).  Mircea Merca, Apr 06 2013
G.f.: W(0), where W(k) = 1 + 2*x*(k+1)/(1  2*x*(k+1)/( 2*x*(k+2) + 1/W(k+1) )); (continued fraction).  Sergei N. Gladkovskii, Aug 28 2013
a(n1) = Sum_{t_1 + 2*t_2 + ... + n*t_n = n} multinomial(t_1 + t_2 + ... + t_n; t_1, t_2, ..., t_n).  Mircea Merca, Dec 06 2013
Construct the power matrix T(n,j) = [A^*j]*[S^*(j1)] where A(n)=(1,1,1,...) and S(n)=(0,1,0,0,...) (where * is convolution operation). Then a(n1) = Sum_{j=1..n} T(n,j).  David Neil McGrath, Jan 01 2015
Exponential convolution of A000012 with themselves.
Sum_{n>=0} a(n)/n! = exp(2) = A072334.
Sum_{n>=0} (1)^n*a(n)/n! = exp(2) = A092553. (End)
G.f.: (r(x) * r(x^2) * r(x^4) * r(x^8) * ...) where r(x) = A090129(x) = (1 + 2x + 2x^2 + 4x^3 + 8x^4 + ...).  Gary W. Adamson, Sep 13 2016
a(n)= n + 1 + Sum_{k=3..n+1} (2*k5)*J(n+2k), where Jacobsthal number J(n) = A001045(n).  Michael A. Allen, Jan 12 2022
Integral_{x=0..Pi} cos(x)^n*cos(n*x) dx = Pi/a(n) (see Nahin, pp. 6970).  Stefano Spezia, May 17 2023


EXAMPLE

There are 2^3 = 8 subsets of a 3element set {1,2,3}, namely { , 1, 2, 3, 12, 13, 23, 123 }.


MAPLE

A000079 := n>2^n; [ seq(2^n, n=0..50) ];
isA000079 := proc(n)
local fs;
fs := numtheory[factorset](n) ;
if n = 1 then
true ;
elif nops(fs) <> 1 then
false;
elif op(1, fs) = 2 then
true;
else
false ;
end if;


MATHEMATICA

Table[2^n, {n, 0, 50}]
CoefficientList[Series[1/(1  2 x), {x, 0, 20}], x] (* Eric W. Weisstein, Sep 21 2017 *)


PROG

(PARI) unimodal(n)=local(x, d, um, umc); umc=0; for (c=0, n!1, x=numtoperm(n, c); d=0; um=1; for (j=2, n, if (x[j]<x[j1], d=1); if (x[j]>x[j1] && d==1, um=0); if (um==0, break)); if (um==1, print(x)); umc+=um); umc
(PARI) x=1; for (n=0, 1000, write("b000079.txt", n, " ", x); x+=x); \\ Harry J. Smith, Apr 26 2009
(Haskell)
a000079 = (2 ^)
a000079_list = iterate (* 2) 1
(Magma) [n le 2 select n else 5*Self(n1)6*Self(n2): n in [1..40]]; // Vincenzo Librandi, Feb 17 2014
(Scala) (List.fill(20)(2: BigInt)).scanLeft(1: BigInt)(_ * _) // Alonso del Arte, Jan 16 2020
(Python)
def a(n): return 1<<n


CROSSREFS

Cf. A000225, A038754, A133464, A140730, A037124, A001787, A001788, A001789, A003472, A054849, A002409, A054851, A140325, A140354, A000041, A152537, A001405, A007318, A000120, A000265, A000593, A001227, A077020, A077021.
The following are parallel families: A000079 (2^n), A004094 (2^n reversed), A028909 (2^n sorted up), A028910 (2^n sorted down), A036447 (double and reverse), A057615 (double and sort up), A263451 (double and sort down); A000244 (3^n), A004167 (3^n reversed), A321540 (3^n sorted up), A321539 (3^n sorted down), A163632 (triple and reverse), A321542 (triple and sort up), A321541 (triple and sort down).


KEYWORD

nonn,core,easy,nice,changed


AUTHOR



EXTENSIONS



STATUS

approved



