cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-6 of 6 results.

A006125 a(n) = 2^(n*(n-1)/2).

Original entry on oeis.org

1, 1, 2, 8, 64, 1024, 32768, 2097152, 268435456, 68719476736, 35184372088832, 36028797018963968, 73786976294838206464, 302231454903657293676544, 2475880078570760549798248448, 40564819207303340847894502572032, 1329227995784915872903807060280344576
Offset: 0

Views

Author

N. J. A. Sloane

Keywords

Comments

Number of graphs on n labeled nodes; also number of outcomes of labeled n-team round-robin tournaments.
Number of perfect matchings of order n Aztec diamond. [see Speyer]
Number of Gelfand-Zeitlin patterns with bottom row [1,2,3,...,n]. [Zeilberger]
For n >= 1 a(n) is the size of the Sylow 2-subgroup of the Chevalley group A_n(2) (sequence A002884). - Ahmed Fares (ahmedfares(AT)my-deja.com), Apr 30 2001
From James Propp: (Start)
a(n) is the number of ways to tile the region
o-----o
|.....|
o--o.....o--o
|...........|
o--o...........o--o
|.................|
o--o.................o--o
|.......................|
|.......................|
|.......................|
o--o.................o--o
|.................|
o--o...........o--o
|...........|
o--o.....o--o
|.....|
o-----o
(top-to-bottom distance = 2n) with dominoes like either of
o--o o-----o
|..| or |.....|
|..| o-----o
|..|
o--o
(End)
The number of domino tilings in A006253, A004003, A006125 is the number of perfect matchings in the relevant graphs. There are results of Jockusch and Ciucu that if a planar graph has a rotational symmetry then the number of perfect matchings is a square or twice a square - this applies to these 3 sequences. - Dan Fux (dan.fux(AT)OpenGaia.com or danfux(AT)OpenGaia.com), Apr 12 2001
Let M_n denotes the n X n matrix with M_n(i,j)=binomial(2i,j); then det(M_n)=a(n+1). - Benoit Cloitre, Apr 21 2002
Smallest power of 2 which can be expressed as the product of n distinct numbers (powers of 2), e.g., a(4) = 1024 = 2*4*8*16. Also smallest number which can be expressed as the product of n distinct powers. - Amarnath Murthy, Nov 10 2002
The number of binary relations that are both reflexive and symmetric on an n-element set. - Justin Witt (justinmwitt(AT)gmail.com), Jul 12 2005
The number of symmetric binary relations on an (n-1)-element set. - Peter Kagey, Feb 13 2021
To win a game, you must flip n+1 heads in a row, where n is the total number of tails flipped so far. Then the probability of winning for the first time after n tails is A005329 / A006125. The probability of having won before n+1 tails is A114604 / A006125. - Joshua Zucker, Dec 14 2005
a(n) = A126883(n-1)+1. - Zerinvary Lajos, Jun 12 2007
Equals right border of triangle A158474 (unsigned). - Gary W. Adamson, Mar 20 2009
a(n-1) is the number of simple labeled graphs on n nodes such that every node has even degree. - Geoffrey Critzer, Oct 21 2011
a(n+1) is the number of symmetric binary matrices of size n X n. - Nathan J. Russell, Aug 30 2014
Let T_n be the n X n matrix with T_n(i,j) = binomial(2i + j - 3, j-1); then det(T_n) = a(n). - Tony Foster III, Aug 30 2018
k^(n*(n-1)/2) is the determinant of n X n matrix T_(i,j) = binomial(k*i + j - 3, j-1), in this case k=2. - Tony Foster III, May 12 2019
Let B_n be the n+1 X n+1 matrix with B_n(i, j) = Sum_{m=max(0, j-i)..min(j, n-i)} (binomial(i, j-m) * binomial(n-i, m) * (-1)^m), 0<=i,j<=n. Then det B_n = a(n+1). Also, deleting the first row and any column from B_n results in a matrix with determinant a(n). The matrices B_n have the following property: B_n * [x^n, x^(n-1) * y, x^(n-2) * y^2, ..., y^n]^T = [(x-y)^n, (x-y)^(n-1) * (x+y), (x-y)^(n-2) * (x+y)^2, ..., (x+y)^n]^T. - Nicolas Nagel, Jul 02 2019
a(n) is the number of positive definite (-1,1)-matrices of size n X n. - Eric W. Weisstein, Jan 03 2021
a(n) is the number of binary relations on a labeled n-set that are both total and antisymmetric. - José E. Solsona, Feb 05 2023

Examples

			From _Gus Wiseman_, Feb 11 2021: (Start)
This sequence counts labeled graphs on n vertices. For example, the a(0) = 1 through a(2) = 8 graph edge sets are:
  {}  {}  {}    {}
          {12}  {12}
                {13}
                {23}
                {12,13}
                {12,23}
                {13,23}
                {12,13,23}
This sequence also counts labeled graphs with loops on n - 1 vertices. For example, the a(1) = 1 through a(3) = 8 edge sets are the following. A loop is represented as an edge with two equal vertices.
  {}  {}    {}
      {11}  {11}
            {12}
            {22}
            {11,12}
            {11,22}
            {12,22}
            {11,12,22}
(End)

References

  • Miklos Bona, editor, Handbook of Enumerative Combinatorics, CRC Press, 2015, page 547 (Fig. 9.7), 573.
  • G. Everest, A. van der Poorten, I. Shparlinski, and T. Ward, Recurrence Sequences, Amer. Math. Soc., 2003; p. 178.
  • J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 517.
  • F. Harary, Graph Theory. Addison-Wesley, Reading, MA, 1969, p. 178.
  • F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 3, Eq. (1.1.2).
  • J. Propp, Enumeration of matchings: problems and progress, in: New perspectives in geometric combinatorics, L. Billera et al., eds., Mathematical Sciences Research Institute series, vol. 38, Cambridge University Press, 1999.
  • N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

Crossrefs

Cf. A000568 for the unlabeled analog, A053763, A006253, A004003.
Cf. A001187 (connected labeled graphs).
Cf. A095340, A103904, A005329, A114604, A299998.
Cf. A158474. - Gary W. Adamson, Mar 20 2009
Cf. A136652 (log). - Paul D. Hanna, Dec 04 2009
The unlabeled version is A000088, or A002494 without isolated vertices.
The directed version is A002416.
The covering case is A006129.
The version for hypergraphs is A058891, or A016031 without singletons.
Row sums of A143543.
The case of connected edge set is A287689.
Cf. A000169, A000295, A036442, A059167, A100743, A136284, A327078.

Programs

Formula

Sequence is given by the Hankel transform of A001003 (Schroeder's numbers) = 1, 1, 3, 11, 45, 197, 903, ...; example: det([1, 1, 3, 11; 1, 3, 11, 45; 3, 11, 45, 197; 11, 45, 197, 903]) = 2^6 = 64. - Philippe Deléham, Mar 02 2004
a(n) = 2^floor(n^2/2)/2^floor(n/2). - Paul Barry, Oct 04 2004
G.f. satisfies: A(x) = 1 + x*A(2x). - Paul D. Hanna, Dec 04 2009
a(n) = 2 * a(n-1)^2 / a(n-2). - Michael Somos, Dec 30 2012
G.f.: G(0)/x - 1/x, where G(k) = 1 + 2^(k-1)*x/(1 - 1/(1 + 1/G(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Jul 26 2013
E.g.f. satisfies A'(x) = A(2x). - Geoffrey Critzer, Sep 07 2013
Sum_{n>=1} 1/a(n) = A299998. - Amiram Eldar, Oct 27 2020
a(n) = s_lambda(1,1,...,1) where s is the Schur polynomial in n variables and lambda is the partition (n,n-1,n-2,...,1). - Leonid Bedratyuk, Feb 06 2022
a(n) = Product_{1 <= j <= i <= n-1} (i + j)/(2*i - 2*j + 1). Cf. A007685. - Peter Bala, Oct 25 2024

Extensions

More terms from Vladeta Jovovic, Apr 09 2000

A028361 Number of totally isotropic spaces of index n in orthogonal geometry of dimension 2n.

Original entry on oeis.org

1, 2, 6, 30, 270, 4590, 151470, 9845550, 1270075950, 326409519150, 167448083323950, 171634285407048750, 351678650799042888750, 1440827432323678715208750, 11804699153027899713705288750, 193419995622362136809061156168750, 6338179836549184861096125026493768750
Offset: 0

Views

Author

N. J. A. Sloane

Keywords

Comments

These numbers appear in first column of A155103. - Mats Granvik, Jan 20 2009
Equals row sums of unsigned triangle A158474. - Gary W. Adamson, Mar 20 2009
a(n) = (n+2) terms in the sequence (1, 1, 2, 4, 8, 16, ...) dot (n+2) terms in the sequence (1, 1, 2, 6, 30, 270, ...). Example: a(4) = 4590 = (1, 2, 4, 8, 16) dot (1, 1, 2, 6, 30, 270) = (1 + 1 + 4 + 24 + 240 + 4320). - Gary W. Adamson, Aug 02 2010
a(n) is the right-hand side of the mass formula used to classify Type II Self Dual Binary Linear Codes of length 2(n+1). a(n) is the number of distinct Type II Self Dual Binary Linear codes of length 2(n+1) when 2(n+1) = 0 MOD 8. It is important to note that Type II codes are only possible when the length is a multiple of 8. In short, this sequence only applies to Type II codes when 2(n+1) = 0 MOD 8, else the right hand side of the mass formula is zero. - Nathan J. Russell, Mar 04 2016
This is almost certainly the sequence of number of Carlyle circles needed for the construction of regular polygons using straightedge and compass mentioned on page 107 of DeTemple (1991). - N. J. A. Sloane, Aug 05 2021
a(n) is also the number of Sp(oo, F2)-orbits of V^n, where V is the countable-dimensional symplectic vector space over the two-element field. - Jingjie Yang, Jul 30 2025

References

  • W. Cary Huffman and Vera Pless, Fundamentals of Error Correcting Codes, Cambridge University Press, 2003, Page 366. - Nathan J. Russell, Mar 04 2016

Links

Crossrefs

Cf. A006125, A028362, A155103, A158474, A323716 (product of 3^i+1).

Programs

  • Magma
    [1] cat [ (&*[1+2^j: j in [0..n-1]]): n in [1..20]]; // G. C. Greubel, Jun 06 2020
  • Maple
    seq( mul((1+2^j), j=0..n-1), n = 0..20); # G. C. Greubel, Jun 06 2020
  • Mathematica
    Table[QPochhammer[-1, 2, n], {n, 0, 15}] (* Arkadiusz Wesolowski, Oct 29 2012 *)
Table[Product[2^i + 1, {i, 0, n/2 - 2}], {n, 2, 32, 2}] (* Nathan J. Russell, Mar 04 2016 *)
Table[Product[2^i + 1, {i, 0, n - 1}], {n, 0, 15}] (* Nathan J. Russell, Mar 04 2016 *)
FoldList[Times,1,2^Range[0,20]+1] (* Harvey P. Dale, Apr 11 2016 *)
  • PARI
    {a(n) = prod(k=0, n-1, 2^k + 1)};
  • PARI
    {a(n)=polcoeff(sum(m=0,n,2^(m*(m-1)/2)*x^m/prod(k=0,m,1-2^k*x+x*O(x^n))),n)} /* Paul D. Hanna, May 02 2012 */
  • Python
    for n in range(2,50,2):
  product = 1
  for i in range(0,n//2-2 + 1):
    product *= (2**i+1)
  print(product)
# Nathan J. Russell, Mar 01 2016
  • Sage
    [product( 1+2^j for j in (0..n-1)) for n in (0..20)] # G. C. Greubel, Jun 06 2020

Formula

a(n) = Product_{i=0..n-1} ( 2^i + 1 ).
Asymptotic to C*2^(n*(n-1)/2) where C = A081845 = 4.76846205806274344829979857... = Product_{k>=0} (1 + 1/2^k). - Benoit Cloitre, Apr 09 2003
It appears that a(n) = 2^((1/2)*(n - 1)*n) * Product_{k>=0} (1 + 1/(2^k)) / Product_{k>=0} (1 + 1/(2^(n + k))). - Peter Moxey (pmoxey(AT)live.com), Mar 21 2010
G.f.: Sum_{n>=0} 2^(n*(n-1)/2) * x^n / Product_{k=0..n} (1 - 2^k*x). - Paul D. Hanna, May 02 2012
a(n) = (a(n-2)^3 + a(n-1) * a(n-3) * (a(n-1) - 2 * a(n-2))) * a(n-1) / (a(n-2)^2 * (a(n-2) - a(n-3))) if n>2. - Michael Somos, Aug 21 2012
0 = a(n)*(+a(n+1) + a(n+2)) + a(n+1)*(-2*a(n+1)) for all n>=0. - Michael Somos, Oct 10 2014
Sum_{k=0..n} 2^k/a(k) = 3-2/a(n) and Sum_{k=0..n} 4^k/a(k) = 9-(4*(1+2^n))/a(n) for n >= 0. - Werner Schulte, Dec 25 2016
G.f. A(x) satisfies: A(x) = (1 + x * A(2*x)) / (1 - x). - Ilya Gutkovskiy, Jun 06 2020
a(n) = Sum_{k=0..n} q_binomial(n, k, q=2) * 2^(k*(k-1)/2). - Jingjie Yang, Jul 30 2025

A135950 Matrix inverse of triangle A022166.

Original entry on oeis.org

1, -1, 1, 2, -3, 1, -8, 14, -7, 1, 64, -120, 70, -15, 1, -1024, 1984, -1240, 310, -31, 1, 32768, -64512, 41664, -11160, 1302, -63, 1, -2097152, 4161536, -2731008, 755904, -94488, 5334, -127, 1, 268435456, -534773760, 353730560, -99486720, 12850368, -777240, 21590, -255, 1
Offset: 0

Views

Author

Paul D. Hanna, Dec 08 2007

Keywords

Comments

A022166 is the triangle of Gaussian binomial coefficients [n,k] for q = 2.
The coefficient [x^k] of Product_{i=1..n} (x-2^(i-1)). - Roger L. Bagula, Mar 20 2009
Triangle T(n,k), 0 <= k <= n, read by rows given by (-1, 1-q, -q^2, q-q^3, -q^4, q^2-q^5, -q^6, q^3-q^7, -q^8, ...) DELTA (1, 0, q, 0, q^2, 0, q^3, 0, q^4, 0, ...) (for q = 2) = (-1, -1, -4, -6, -16, -28, -64, -120, -256, ...) DELTA (1, 0, 2, 0, 4, 0, 8, 0, 16, 0, ...) where DELTA is the operator defined in A084938. - Philippe Deléham, Oct 20 2013
Reversed rows of triangle A158474. - Werner Schulte, Apr 06 2019
T(n,k) = Sum mu(0,U) where the sum is taken over the subspaces U of GF(2)^n having dimension n-k and mu is the Moebius function of the poset of all subspaces of GF(2)^n. - Geoffrey Critzer, Jun 02 2024

Examples

			Triangle begins:
         1;
        -1,       1;
         2,      -3,        1;
        -8,      14,       -7,      1;
        64,    -120,       70,    -15,      1;
     -1024,    1984,    -1240,    310,    -31,    1;
     32768,  -64512,    41664, -11160,   1302,  -63,    1;
  -2097152, 4161536, -2731008, 755904, -94488, 5334, -127, 1; ...

Links

Crossrefs

Cf. A022166, A006125, A028361, A127850, A135951 (central terms), A158474.

Programs

  • Mathematica
    max = 9; M = Table[QBinomial[n, k, 2], {n, 0, max}, {k, 0, max}] // Inverse; Table[M[[n, k]], {n, 1, max+1}, {k, 1, n}] // Flatten (* Jean-François Alcover, Apr 08 2016 *)
p[x_, n_, q_] := (-1)^n*q^Binomial[n, 2]*QPochhammer[x, 1/q, n];
Table[CoefficientList[Series[p[x, n, 2], {x, 0, n}], x], {n, 0, 10}]// Flatten (* G. C. Greubel, Apr 15 2019 *)
  • PARI
    T(n,k)=local(q=2,A=matrix(n+1,n+1,n,k,if(n>=k,if(n==1 || k==1, 1, prod(j=n-k+1, n-1, 1-q^j)/prod(j=1, k-1, 1-q^j))))^-1);A[n+1,k+1]

Formula

Unsigned column 0 equals A006125(n) = 2^(n*(n-1)/2).
Unsigned column 1 equals A127850(n) = (2^n-1)*2^(n*(n-1)/2)/2^(n-1).
Row sums equal 0^n.
Unsigned row sums equal A028361(n) = Product_{k=0..n} (1+2^k).
T(n,k) = (-1)^(n-k) * A022166(n,k) * 2^binomial(n-k,2) for 0 <= k <= n. - Werner Schulte, Apr 06 2019 [corrected by Werner Schulte, Dec 27 2021]
Sum_{n>=0} Sum_{k=0..n} T(n,k)y^k*x^n/A005329(n) = e(y*x)/e(x) where e(x) = Sum_{n>=0} x^n/A005329(n). - Geoffrey Critzer, Jun 02 2024

A157963 Triangle T(n,k), 0<=k<=n, read by rows given by [1,q-1,q^2,q^3-q,q^4,q^5-q^2,q^6,q^7-q^3,q^8,...] DELTA [ -1,0,-q,0,-q^2,0,-q^3,0,-q^4,0,-q^5,0,...] (for q=2) = [1,1,4,6,16,28,64,...] DELTA [ -1,0,-2,0,-4,0,-8,0,-16,0,...] where DELTA is the operator defined in A084938.

Original entry on oeis.org

1, 1, -1, 2, -3, 1, 8, -14, 7, -1, 64, -120, 70, -15, 1, 1024, -1984, 1240, -310, 31, -1, 32768, -64512, 41664, -11160, 1302, -63, 1, 2097152, -4161536, 2731008, -755904, 94488, -5334, 127, -1, 268435456, -534773760, 353730560, -99486720, 12850368
Offset: 0

Views

Author

Philippe Deléham, Mar 10 2009

Keywords

Comments

Row sums equal 0^n.
Row n contains the coefficients of Product_{j=0..n-1} (2^j*x-1), highest coefficient first. - Alois P. Heinz, Mar 26 2012
The elements of the matrix inverse are apparently given by T^(-1)(n,k) = (-1)^k*A022166(n,k). - R. J. Mathar, Mar 26 2013

Examples

			Triangle begins :
1;
1,    -1;
2,    -3,  1;
8,   -14,  7,  -1;
64, -120, 70, -15,  1;

Links

Crossrefs

Cf. A007179, A130595, A157783, A157784, A157785, A157832, A158474.

Programs

  • Maple
    T:= n-> seq (coeff (mul (2^j*x-1, j=0..n-1), x, n-k), k=0..n):
seq (T(n), n=0..10);  # Alois P. Heinz, Mar 26 2012
  • Mathematica
    row[n_] := CoefficientList[(-1)^n QPochhammer[x, 2, n] + O[x]^(n+1), x] // Reverse; Table[row[n], {n, 0, 10}] // Flatten (* Jean-François Alcover, May 26 2016 *)

Formula

T(n,k) = (-1)^n*A135950(n,k). T(n,0) = A006125(n).
T(n,k) = [x^(n-k)] Product_{j=0..n-1} (2^j*x-1). - Alois P. Heinz, Mar 26 2012

A305737 Number of subsets S of vectors in GF(2)^n such that span(S) = GF(2)^n.

Original entry on oeis.org

1, 2, 8, 184, 62464, 4293001088, 18446743803209556992, 340282366920938461120638132973980614656, 115792089237316195423570985008687907766497981100801256254562260326801824546816
Offset: 0

Views

Author

Geoffrey Critzer, Jun 22 2018

Keywords

Comments

Asymptotic to A001146(n) = 2^(2^n).

References

  • R. P. Stanley, Enumerative Combinatorics Vol 1, Cambridge, 1997, page 127.

Links

Crossrefs

Cf. A001146, A022166, A158474, A051179.

Programs

  • Mathematica
    Table[Sum[QBinomial[n, k, q] (-1)^(n - k) q^Binomial[n - k, 2] (2^(q^k) - 1) /. q -> 2, {k, 0, n}], {n, 0, 8}]
  • PARI
    \\ here U(n,k) is A022166(n,k).
U(n,k)={polcoeff(x^k/prod(j=0, k, 1-2^j*x+x*O(x^n)), n)}
a(n)={sum(k=0, n, U(n,k)*(-1)^(n-k)*2^binomial(n-k,2)*(2^(2^k)-1))} \\ Andrew Howroyd, Mar 01 2020

Formula

a(n) = Sum_{k=0..n} A022166(n,k)*(-1)^(n-k)*2^binomial(n-k,2)*(2^(2^k)-1).
Sum_{k=0..n} a(k)* A022166(n,k) = 2^(2^n) - 1. - Geoffrey Critzer, Apr 25 2024
a(n) = Sum_{k=0..n} A158474(n,k) * A051179(n-k). - Tilman Piesk, Mar 12 2025

Extensions

a(8) corrected by Andrew Howroyd, Mar 01 2020

A194583 Triangle T(n,k) with T(n,0)=1 and T(n,k) = (2^(n+1)-2^k)*T(n,k-1) + T(n+1,k-1) otherwise.

Original entry on oeis.org

1, 1, 3, 1, 7, 43, 1, 15, 211, 2619, 1, 31, 931, 26251, 654811, 1, 63, 3907, 234795, 13255291, 662827803, 1, 127, 16003, 1985131, 238658491, 26961325147, 2699483026843, 1, 255, 64771, 16323819, 4050110011, 973958217435, 220115609012251, 44102911693372059, 1, 511, 260611, 132393451, 66733574971, 33115631264731, 15928113739803931, 7200501591899676571, 2886238576935227688091
Offset: 0

Views

Author

R. J. Mathar, Aug 29 2011

Keywords

Examples

			The triangle starts in row n=0 as
  1;
  1,  3;
  1,  7,  43;
  1, 15, 211,  2619;
  1, 31, 931, 26251, 654811;

Links

Programs

  • Maple
    A194583 := proc(n,k) option remember; if n=0 or k=0 then 1; elif k> n then
return procname(k,n); else (2^(n+1)-2^k)*procname(n,k-1)+procname(n+1,k-1) ; end if;
end proc:
  • Mathematica
    t[, 0] = 1; t[n, k_] := t[n, k] = (2^(n+1)-2^k)*t[n, k-1]+t[n+1, k-1]; Table[t[n, k], {n, 0, 8}, {k, 0, n}] // Flatten (* Jean-François Alcover, Jan 10 2014 *)

Formula

T(n,1) = A000225(n+1).
T(n,2) = (2^(n+1)-4)*(2^(n+1)-1) + 2^(n+2) - 1.
T(n,k) = -Sum_{j=1..k+1} A158474(k+1,j)*T(n-j,k) assuming the symmetric extension T(n,k) = T(k,n).
Showing 1-6 of 6 results.

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