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.

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A324176 Integers k such that floor(sqrt(k)) + floor(sqrt(k/3)) divides k.

Original entry on oeis.org

1, 2, 6, 15, 18, 24, 32, 36, 45, 55, 72, 78, 84, 98, 105, 112, 136, 144, 152, 180, 198, 220, 230, 275, 336, 390, 403, 462, 525, 540, 608, 663, 697, 756, 774, 792, 836, 855, 874, 940, 980, 1050, 1092, 1144, 1166, 1265, 1368, 1392, 1500, 1525, 1586, 1638, 1755, 1782, 1848, 1904
Offset: 1

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Author

Jinyuan Wang, Mar 08 2019

Keywords

Comments

This sequence is infinite for the same reason that A324175 is: if x-1 > y > 1 satisfies x^2 - 3*y^2 = -2 (x=A001834(j), y=A001835(j+1), j>0), then x < 3*y. Let k = 3*y^2 + m. By the pigeonhole principle there exists a number m belonging to [0, 2*x - 2] such that x + y | 3*y^2 + m, so such a k is a term.

Crossrefs

Programs

  • Mathematica
    Select[Range[2000],Divisible[#,Floor[Sqrt[#]]+Floor[Sqrt[#/3]]]&] (* Harvey P. Dale, Jun 19 2021 *)
  • PARI
    is(n) = n%(floor(sqrt(n)) + floor(sqrt(n/3))) == 0;

A001352 a(0) = 1, a(1) = 6, a(2) = 24; for n>=3, a(n) = 4a(n-1) - a(n-2).

Original entry on oeis.org

1, 6, 24, 90, 336, 1254, 4680, 17466, 65184, 243270, 907896, 3388314, 12645360, 47193126, 176127144, 657315450, 2453134656, 9155223174, 34167758040, 127515808986, 475895477904, 1776066102630, 6628368932616, 24737409627834, 92321269578720, 344547668687046
Offset: 0

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Author

Keywords

Comments

Also the coordination sequence of a {4,6} tiling of the hyperbolic plane, where there are 6 squares (with vertex angles Pi/3) around every vertex. - toen (tca110(AT)rsphysse.anu.edu.au), May 16 2005
a(n) is related to the almost-equilateral Heronian triangles because it is the area of the Heronian triangle with edge lengths A003500(n)-1, A003500(n)+1 and 4. - Herbert Kociemba, Mar 19 2021

References

  • Bastida, Julio R. Quadratic properties of a linearly recurrent sequence. Proceedings of the Tenth Southeastern Conference on Combinatorics, Graph Theory and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1979), pp. 163--166, Congress. Numer., XXIII-XXIV, Utilitas Math., Winnipeg, Man., 1979. MR0561042 (81e:10009) - From N. J. A. Sloane, May 30 2012
  • 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).

Crossrefs

First differences of A082841. Pairwise sums of A001834.

Programs

  • Maple
    A001352 := proc(n) coeftayl(1+6*x/(1-4*x+x^2),x=0,n) ; end: for n from 0 to 30 do printf("%d,",A001352(n)) ; od ; # R. J. Mathar, Jun 06 2007
    A001352:=(z+1)**2/(1-4*z+z**2); # conjectured by Simon Plouffe in his 1992 dissertation
  • Mathematica
    Join[{1},LinearRecurrence[{4,-1},{6,24},30]] (* Harvey P. Dale, Jul 20 2011 *)
  • PARI
    Vec((x+1)^2/(x^2-4*x+1) + O(x^40)) \\ Colin Barker, Oct 12 2015

Formula

G.f.: 1+6x/(1-4x+x^2). - R. J. Mathar, Jun 06 2007
a(n) = sqrt(3)*(-(2-sqrt(3))^n+(2+sqrt(3))^n) for n>0. - Colin Barker, Oct 12 2015

Extensions

More terms from R. J. Mathar, Jun 06 2007

A121338 Pentagonal numbers P(k) that are one-third of other pentagonal numbers: P(k) such that 3*P(k)=P(m) for some m>k.

Original entry on oeis.org

70, 511258935, 3732600255368600, 27250975409595074561065, 198953975772318806945317308330, 1452523584226469439408576900215922395, 10604587088767577582197244731443261336155260, 77421990626847055423676582260371016672624778798925
Offset: 1

Views

Author

Franz Vrabec, Aug 28 2006

Keywords

Comments

The k values are (A001835(6n-2)+1)/6, the m values are (A001834(6n-3)+1)/6.

Examples

			a(1) = ((A001835(4))^2-1)/24 = (41^2-1)/24 = 70; this number and 3*70=210 are pentagonal numbers (in A000326).
		

Crossrefs

Programs

  • Mathematica
    CoefficientList[Series[5 (x^2 + 40545 x + 14)/((1 - x) (x^2 - 7300802 x + 1)), {x, 0, 20}], x] (* Vincenzo Librandi, Jun 21 2015 *)
  • PARI
    Vec(-5*x*(x^2+40545*x+14)/((x-1)*(x^2-7300802*x+1)) + O(x^20)) \\ Colin Barker, Jun 20 2015

Formula

a(n) = ((A001835(6n-2))^2-1)/24.
a(n) = 7300803*a(n-1)-7300803*a(n-2)+a(n-3). - Colin Barker, Jun 20 2015
G.f.: -5*x*(x^2+40545*x+14) / ((x-1)*(x^2-7300802*x+1)). - Colin Barker, Jun 20 2015

Extensions

Added more terms, Colin Barker, Jun 20 2015

A207607 Triangle of coefficients of polynomials v(n,x) jointly generated with A207606; see Formula section.

Original entry on oeis.org

1, 1, 2, 1, 5, 2, 1, 9, 9, 2, 1, 14, 25, 13, 2, 1, 20, 55, 49, 17, 2, 1, 27, 105, 140, 81, 21, 2, 1, 35, 182, 336, 285, 121, 25, 2, 1, 44, 294, 714, 825, 506, 169, 29, 2, 1, 54, 450, 1386, 2079, 1716, 819, 225, 33, 2, 1, 65, 660, 2508, 4719, 5005, 3185, 1240, 289, 37, 2
Offset: 1

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Author

Clark Kimberling, Feb 19 2012

Keywords

Comments

Subtriangle of the triangle T(n,k) given by (1, 0, 1/2, 1/2, 0, 0, 0, 0, 0, 0, 0, ...) DELTA (0, 2, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...) where DELTA is the operator defined in A084938. - Philippe Deléham, Mar 03 2012

Examples

			First five rows:
  1;
  1,  2;
  1,  5,  2;
  1,  9,  9,  2;
  1, 14, 25, 13,  2;
Triangle (1, 0, 1/2, 1/2, 0, 0, 0, ...) DELTA (0, 2, -1, 0, 0, 0, 0, ...) begins:
  1;
  1,  0;
  1,  2,  0;
  1,  5,  2,  0;
  1,  9,  9,  2,  0;
  1, 14, 25, 13,  2,  0;
  1, 20, 55, 49, 17,  2,  0;
  ...
1 = 2*1 - 1, 20 = 2*14 + 1 - 9, 55 = 2*25 + 14 - 9, 49 = 2*13 + 25 - 2, 17 = 2*2 + 1 - 0, 2 = 2*0 + 2 - 0. - _Philippe Deléham_, Mar 03 2012
		

Crossrefs

Cf. A207606.

Programs

  • Maple
    A207607:= (n,k) -> `if`(k=1, 1, binomial(n+k-3, 2*k-2) + 2*binomial(n+k-3, 2*k-3) ); seq(seq(A207607(n, k), k = 1..n), n = 1..10); # G. C. Greubel, Mar 15 2020
  • Mathematica
    (* First program *)
    u[1, x_] := 1; v[1, x_] := 1; z = 16;
    u[n_, x_] := u[n - 1, x] + v[n - 1, x]
    v[n_, x_] := x*u[n - 1, x] + (x + 1)*v[n - 1, x]
    Table[Factor[u[n, x]], {n, 1, z}]
    Table[Factor[v[n, x]], {n, 1, z}]
    cu = Table[CoefficientList[u[n, x], x], {n, 1, z}];
    TableForm[cu]
    Flatten[%]  (* A207606 *)
    Table[Expand[v[n, x]], {n, 1, z}]
    cv = Table[CoefficientList[v[n, x], x], {n, 1, z}];
    TableForm[cv]
    Flatten[%]  (* A207607 *)
    (* Second program *)
    Table[If[k==1, 1, Binomial[n+k-3, 2*k-2] + 2*Binomial[n+k-3, 2*k-3]], {n, 10}, {k, n}]//Flatten (* G. C. Greubel, Mar 15 2020 *)
  • Python
    from sympy import Poly
    from sympy.abc import x
    def u(n, x): return 1 if n==1 else u(n - 1, x) + v(n - 1, x)
    def v(n, x): return 1 if n==1 else x*u(n - 1, x) + (x + 1)*v(n - 1, x)
    def a(n): return Poly(v(n, x), x).all_coeffs()[::-1]
    for n in range(1, 13): print(a(n)) # Indranil Ghosh, May 28 2017
    
  • Sage
    def T(n, k):
        if k == 1: return 1
        else: return binomial(n+k-3, 2*k-2) + 2*binomial(n+k-3, 2*k-3)
    [[T(n, k) for k in (1..n)] for n in (1..12)] # G. C. Greubel, Mar 15 2020

Formula

u(n,x) = u(n-1,x) + v(n-1,x), v(n,x) = x*u(n-1,x) + (x+1)v(n-1,x), where u(1,x)=1, v(1,x)=1.
T(n,k) = 2*T(n-1,k) + T(n-1,k-1) - T(n-2,k). - Philippe Deléham, Mar 03 2012
G.f.: (1-x+y*x)/(1-(y+2)*x+x^2). - Philippe Deléham, Mar 03 2012
For n >= 1, Sum{k=0..n} T(n,k)*x^k = A000012(n), A001906(n), A001834(n-1), A055271(n-1), A038761(n-1), A056914(n-1) for x = 0, 1, 2, 3, 4, 5 respectively. - Philippe Deléham, Mar 03 2012
T(n,k) = C(n+k-1,2*k) + 2*C(n+k-1,2*k-1). where C is binomial. - Yuchun Ji, May 23 2019
T(n,k) = T(n-1,k) + A207606(n,k-1). - Yuchun Ji, May 28 2019
Sum_{k=1..n} T(n, k)*x^k = { 4*(-1)^(n-1)*A016921(n-1) (x=-4), 3*(-1)^(n-1) * A130815(n-1) (x=-3), 2*(-1)^(n-1)*A010684(n-1) (x=-2), A057079(n+1) (x=-1), 0 (x=0), A001906(n) = Fibonacci(2*n) (x=1), 2*A001834(n-1) (x=2), 3*A055271(n-1) (x=3), 4*A038761(n-1) (x=4) }. - G. C. Greubel, Mar 15 2020

A209759 Triangle of coefficients of polynomials u(n,x) jointly generated with A209760; see the Formula section.

Original entry on oeis.org

1, 1, 2, 1, 5, 5, 1, 5, 16, 13, 1, 5, 19, 48, 34, 1, 5, 19, 68, 141, 89, 1, 5, 19, 71, 233, 409, 233, 1, 5, 19, 71, 262, 772, 1175, 610, 1, 5, 19, 71, 265, 948, 2492, 3349, 1597, 1, 5, 19, 71, 265, 986, 3354, 7879, 9482, 4181, 1, 5, 19, 71, 265, 989, 3641
Offset: 1

Views

Author

Clark Kimberling, Mar 14 2012

Keywords

Comments

Limiting row: A001834
Coefficient of x^n in u(n,x): odd-indexed Fibonacci numbers
Alternating row sums: 1,-1,1,-1,1,-1,1,-1,...; A033999
For a discussion and guide to related arrays, see A208510.

Examples

			First five rows:
1
1...2
1...5...5
1...5...16...13
1...5...19...48...34
First three polynomials u(n,x): 1, 1 + 2x, 1 + 5x + 5x^2.
		

Crossrefs

Programs

  • Mathematica
    u[1, x_] := 1; v[1, x_] := 1; z = 16;
    u[n_, x_] := x*u[n - 1, x] + (x + 1)*v[n - 1, x];
    v[n_, x_] := x*u[n - 1, x] + 2 x*v[n - 1, x] + 1;
    Table[Expand[u[n, x]], {n, 1, z/2}]
    Table[Expand[v[n, x]], {n, 1, z/2}]
    cu = Table[CoefficientList[u[n, x], x], {n, 1, z}];
    TableForm[cu]
    Flatten[%]    (* A209759 *)
    Table[Expand[v[n, x]], {n, 1, z}]
    cv = Table[CoefficientList[v[n, x], x], {n, 1, z}];
    TableForm[cv]
    Flatten[%]    (* A209760 *)

Formula

u(n,x)=x*u(n-1,x)+(x+1)*v(n-1,x),
v(n,x)=x*u(n-1,x)+2x*v(n-1,x)+1,
where u(1,x)=1, v(1,x)=1.

A210224 Triangle of coefficients of polynomials v(n,x) jointly generated with A210223; see the Formula section.

Original entry on oeis.org

1, 1, 3, 1, 5, 5, 1, 5, 15, 7, 1, 5, 19, 35, 9, 1, 5, 19, 63, 69, 11, 1, 5, 19, 71, 177, 121, 13, 1, 5, 19, 71, 249, 429, 195, 15, 1, 5, 19, 71, 265, 781, 923, 295, 17, 1, 5, 19, 71, 265, 957, 2171, 1807, 425, 19, 1, 5, 19, 71, 265, 989, 3211, 5407, 3281, 589, 21
Offset: 1

Views

Author

Clark Kimberling, Mar 20 2012

Keywords

Comments

Limiting row: A001834
For a discussion and guide to related arrays, see A208510.

Examples

			First five rows:
1
1...3
1...5...5
1...5...15...7
1...5...19...35...9
First three polynomials v(n,x): 1, 1 + 3x , 1 + 5x + 5x^2.
		

Crossrefs

Programs

  • Mathematica
    u[1, x_] := 1; v[1, x_] := 1; z = 16;
    u[n_, x_] := x*u[n - 1, x] + v[n - 1, x] + 1;
    v[n_, x_] := 2 x*u[n - 1, x] + x*v[n - 1, x] + 1;
    Table[Expand[u[n, x]], {n, 1, z/2}]
    Table[Expand[v[n, x]], {n, 1, z/2}]
    cu = Table[CoefficientList[u[n, x], x], {n, 1, z}];
    TableForm[cu]
    Flatten[%]     (* A210223 *)
    Table[Expand[v[n, x]], {n, 1, z}]
    cv = Table[CoefficientList[v[n, x], x], {n, 1, z}];
    TableForm[cv]
    Flatten[%]     (* A210224 *)

Formula

u(n,x)=x*u(n-1,x)+v(n-1,x)+1,
v(n,x)=2x*u(n-1,x)+x*v(n-1,x)+1,
where u(1,x)=1, v(1,x)=1.

A386706 Expansion of ((Product_{k>=1} (1 - x^k)^2/(1 - 4*x^k + x^(2k))) - 1)/2.

Original entry on oeis.org

0, 1, 5, 18, 71, 260, 990, 3672, 13775, 51343, 191860, 715770, 2672298, 9972092, 37220040, 138903480, 518408351, 1934712530, 7220497115, 26947209762, 100568547820, 375326739216, 1400739172470, 5227629044040, 19509779871450, 72811487038701, 271736178975820, 1014133216234068
Offset: 0

Views

Author

Christian Kassel, Jul 30 2025

Keywords

Comments

a(n) is the value at q = 2 + sqrt(3) of C_n(q)/(q^{n-1}(q - 1)^2), where C_n(q) is the number of codimension n ideals of the algebra of two-variable Laurent polynomials over a finite field of order q. The number C_n(q) is a palindromic polynomial of degree 2n with integer coefficients in the variable q and it is divisible by (q-1)^2.

Crossrefs

Programs

  • Mathematica
    nmax = 30; CoefficientList[Series[(Product[(1 - x^k)^2/(1 - 4*x^k + x^(2*k)), {k, 1, nmax}] - 1)/2, {x, 0, nmax}], x] (* Vaclav Kotesovec, Jul 30 2025 *)

Formula

G.f.: ((Product_{k>=1} (1 - x^k)^2/(1 - 4*x^k + x^(2k))) - 1)/2.
a(2^k) = A001834(2^k-1) for all nonnegative integers k. Follows from Cor. 4.5 of Kassel-Reutenauer paper "Pairs of intertwined integer sequences".
a(n) ~ (1 + sqrt(3))^(2*n-1) / 2^n. - Vaclav Kotesovec, Jul 30 2025

Extensions

a(0)=0 added, offset changed to 0, a(7) corrected and more terms added by Vaclav Kotesovec, Jul 30 2025

A109438 a(n) = 5a(n-1) - 5a(n-2) + a(n-3) + 2*(-1)^(n+1), alternatively a(n) = 3a(n-1) + 3a(n-2) - a(n-3).

Original entry on oeis.org

1, 5, 18, 68, 253, 945, 3526, 13160, 49113, 183293, 684058, 2552940, 9527701, 35557865, 132703758, 495257168, 1848324913, 6898042485, 25743845026, 96077337620, 358565505453, 1338184684193, 4994173231318, 18638508241080
Offset: 0

Views

Author

Creighton Dement, Jun 28 2005

Keywords

Comments

See A109437 for comments.
Floretion Algebra Multiplication Program, FAMP Code: (-1)^(n)jbasejfor[ + .5'ii' + .5'kk' + .5'ij' + .5'ji' + .5'jk' + .5'kj'] 1vesfor = (-1,-1,-1,-1,)

Crossrefs

Programs

  • Mathematica
    LinearRecurrence[{3,3,-1},{1,5,18},30] (* Harvey P. Dale, Sep 07 2021 *)
  • PARI
    Vec((1 + 2*x) / ((1 + x)*(1 - 4*x + x^2)) + O(x^30)) \\ Colin Barker, May 12 2019

Formula

G.f.: (1+2*x) / ((x+1)*(x^2-4*x+1)).
a(n) = (-2*(-1)^n + (7-5*sqrt(3))*(2-sqrt(3))^n + (2+sqrt(3))^n*(7+5*sqrt(3))) / 12. - Colin Barker, May 12 2019

A129743 a(n) = -(u^n-1)*(v^n-1) with u = 2+sqrt(3), v = 2-sqrt(3).

Original entry on oeis.org

2, 12, 50, 192, 722, 2700, 10082, 37632, 140450, 524172, 1956242, 7300800, 27246962, 101687052, 379501250, 1416317952, 5285770562, 19726764300, 73621286642, 274758382272, 1025412242450, 3826890587532, 14282150107682, 53301709843200, 198924689265122, 742397047217292
Offset: 1

Views

Author

N. J. A. Sloane, May 13 2007

Keywords

Comments

Each term of this sequence beyond the sixth has a primitive prime divisor. - Anthony Flatters (Anthony.Flatters(AT)uea.ac.uk), Aug 17 2007
a(n) is also the number of spanning trees for the n-gear graph. - Eric W. Weisstein, Jul 16 2011

Crossrefs

Programs

  • Maple
    u:=2+sqrt(3): v:=2-sqrt(3): a:=n->expand(-(u^n-1)*(v^n-1)): seq(a(n),n=1..28); # Emeric Deutsch, May 13 2007
  • Mathematica
    Table[-((2 + Sqrt[3])^n - 1)*((2 - Sqrt[3])^n - 1), {n, 30}] // Expand (* Stefan Steinerberger, May 15 2007 *)
    LinearRecurrence[{5, -5, 1}, {2, 12, 50}, 30]
    LucasL[2 Range[20], Sqrt[2]] - 2 // Round (* Eric W. Weisstein, Mar 28 2018 *)
  • PARI
    my(x='x+O('x^30)); Vec(2*x*(1+x)/((1-x)*(1-4*x+x^2))) \\ Altug Alkan, Mar 28 2018

Formula

a(2*n) = 12*A001353(n)^2, a(2*n+1) = 2*A001834(n)^2. - Vladeta Jovovic, May 30 2007
a(n) = 2*A092184(n). - Robert G. Wilson v, Jul 04 2007
O.g.f.: 2*x*(1+x)/((1-x)*(1-4*x+x^2)). - R. J. Mathar, Dec 05 2007
a(n) = 5*a(n-1) - 5*a(n-2) + a(n-3). - Eric W. Weisstein, Jul 15 2011
E.g.f.: 2*exp(x)*(exp(x)*cosh(sqrt(3)*x) - 1). - Stefano Spezia, May 05 2024

Extensions

More terms from Emeric Deutsch and Stefan Steinerberger, May 13 2007
More terms from Vladeta Jovovic, May 30 2007

A133161 Indices of the triangular numbers which are also centered triangular number.

Original entry on oeis.org

1, 4, 16, 61, 229, 856, 3196, 11929, 44521, 166156, 620104, 2314261, 8636941, 32233504, 120297076, 448954801, 1675522129, 6253133716, 23337012736, 87094917229, 325042656181, 1213075707496, 4527260173804, 16895964987721
Offset: 1

Views

Author

Richard Choulet, Oct 09 2007

Keywords

Comments

Also, indices of the triangular numbers which are sums of three consecutive triangular numbers (see A129803).

Crossrefs

Programs

  • Mathematica
    LinearRecurrence[{5,-5,1},{1,4,16},30] (* Harvey P. Dale, Aug 29 2017 *)

Formula

a(n+2)=4*a(n+1)-a(n)+1.
a(n+1)=2*a(n)+0.5+0.5*(12*a(n)^2+12*a(n)-15)^0.5.
G.f.: x*(1-x+x^2)/(1-x)/(1-4*x+x^2). - R. J. Mathar, Oct 24 2007
a(n)-a(n-1)= A005320(n-1). - R. J. Mathar, Mar 14 2016
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