A026961 -id:A026961 - cp's OEIS frontend

A026635 a(n) = Sum_{i=0..n} Sum_{j=0..n} A026626(i,j).

1, 3, 8, 18, 40, 84, 174, 354, 716, 1440, 2890, 5790, 11592, 23196, 46406, 92826, 185668, 371352, 742722, 1485462, 2970944, 5941908, 11883838, 23767698, 47535420, 95070864, 190141754, 380283534, 760567096, 1521134220, 3042268470, 6084536970, 12169073972

Offset: 0

Clark Kimberling

Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-1,-3,2).

Cf. A026626, A026627, A026628, A026629, A026630, A026631, A026632.
Cf. A026633, A026634, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[n eq 0 select 1 else (17*2^n -6*n-9+(-1)^n)/6: n in [0..40]]; // G. C. Greubel, Jun 21 2024

LinearRecurrence[{3,-1,-3,2},{1,3,8,18,40},40] (* Harvey P. Dale, Jan 17 2024 *)

Vec((1 + x^4) / ((1 - x)^2*(1 + x)*(1 - 2*x)) + O(x^40)) \\ Colin Barker, Sep 29 2017

[(17*2^n -6*n -9 +(-1)^n -3*int(n==0))/6 for n in range(41)] # G. C. Greubel, Jun 21 2024

G.f.: (1+x^4)/((1-x)*(1-2*x)*(1-x^2)). - Ralf Stephan, Apr 30 2004
From Colin Barker, Sep 29 2017: (Start)
a(n) = (17*2^n - 6*n - 9 + (-1)^n)/6 for n>0.
a(n) = 3*a(n-1) - a(n-2) - 3*a(n-3) + 2*a(n-4) for n>4. (End)
E.g.f.: (1/6)*(-3 - 3*(3+2*x)*exp(x) + 17*exp(2*x) + exp(-x)). - G. C. Greubel, Jun 21 2024

A026636 a(n) = Sum_{k=0..floor(n/2)} A026626(n-k, k).

Original entry on oeis.org

1, 1, 2, 4, 6, 11, 17, 30, 47, 78, 125, 205, 330, 536, 866, 1404, 2270, 3675, 5945, 9622, 15567, 25190, 40757, 65949, 106706, 172656, 279362, 452020, 731382, 1183403, 1914785, 3098190, 5012975, 8111166, 13124141, 21235309

Offset: 0

Clark Kimberling

G. C. Greubel, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (1,1,0,1,-1,-1).

Cf. A026626, A026627, A026628, A026629, A026630, A026631, A026632.
Cf. A026633, A026634, A026635, A026961, A026962, A026963, A026964.
Cf. A026965.

I:=[2,4,6,11,17,30]; [1,1] cat [n le 6 select I[n] else Self(n-1) +Self(n-2) +Self(n-4) -Self(n-5) -Self(n-6): n in [1..50]]; // G. C. Greubel, Jun 21 2024

LinearRecurrence[{1,1,0,1,-1,-1}, {1,1,2,4,6,11,17,30}, 41] (* G. C. Greubel, Jun 21 2024 *)

@CachedFunction
def a(n): # a = A026636
    if n<8: return (1,1,2,4,6,11,17,30)[n]
    else: return a(n-1) +a(n-2) +a(n-4) -a(n-5) -a(n-6)
[a(n) for n in range(41)] # G. C. Greubel, Jun 21 2024

G.f.: (1 + x^3 - x^4 + x^5 + x^7)/((1-x^4)*(1-x-x^2)).
From G. C. Greubel, Jun 21 2024: (Start)
a(n) = (1/20)*(2*LucasL(n-1) + 70*Fibonacci(n-1) - 15*(1+(-1)^n) - 4*cos((n-1)*Pi/2) - 2*sin((n-1)*Pi/2)) - [n=0] + [n=1].
E.g.f.: (1/10)*(cos(x) - 2*sin(x) - 15*cosh(x) - 10*(1 - x) + 2*exp(x/2)*(17*cosh(sqrt(5)*x/2) - 3*sqrt(5)*sinh(sqrt(5)*x/2))). (End)

A026626 Triangular array T read by rows: T(n,0) = T(n,n) = 1 for n >= 0, T(n,1) = T(n,n-1) = floor(3*n/2) for n >= 1, T(n,k) = T(n-1,k-1) + T(n-1,k) for 2 <= k <= n-2, n >= 4.

Original entry on oeis.org

1, 1, 1, 1, 3, 1, 1, 4, 4, 1, 1, 6, 8, 6, 1, 1, 7, 14, 14, 7, 1, 1, 9, 21, 28, 21, 9, 1, 1, 10, 30, 49, 49, 30, 10, 1, 1, 12, 40, 79, 98, 79, 40, 12, 1, 1, 13, 52, 119, 177, 177, 119, 52, 13, 1, 1, 15, 65, 171, 296, 354, 296, 171, 65, 15, 1, 1, 16, 80, 236, 467, 650, 650, 467, 236, 80, 16, 1

Offset: 0

Clark Kimberling

			Triangle begins as:
  1;
  1,  1;
  1,  3,  1;
  1,  4,  4,   1;
  1,  6,  8,   6,   1;
  1,  7, 14,  14,   7,   1;
  1,  9, 21,  28,  21,   9,   1;
  1, 10, 30,  49,  49,  30,  10,   1;
  1, 12, 40,  79,  98,  79,  40,  12,   1;
  1, 13, 52, 119, 177, 177, 119,  52,  13,   1;
  1, 15, 65, 171, 296, 354, 296, 171,  65,  15,  1;

G. C. Greubel, Rows n = 0..50 of the triangle, flattened
Index entries for sequences related to rooted trees

Cf. A006578, A026627, A026628, A026629, A026630, A026631, A026632.
Cf. A026633, A026634, A026635, A026636, A026961, A026962, A026963.
Cf. A026964, A026965, A032766, A084056.

function T(n,k) // T = A026626
  if k eq 0 or k eq n then return 1;
  elif k eq 1 or k eq n-1 then return Floor(3*n/2);
  else return T(n-1,k-1) + T(n-1,k);
  end if;
end function;
[T(n,k): k in [0..n], n in [0..12]]; // G. C. Greubel, Jun 19 2024

T[n_, k_]:= T[n, k]= If[k==0 || k==n, 1, If[k==1 || k==n-1, (6*n-1+(-1)^n)/4, T[n-1, k-1] + T[n-1, k] ]];
Table[T[n,k], {n,0,12}, {k,0,n}]//Flatten (* G. C. Greubel, Jun 19 2024 *)

def T(n,k): # T = A026626
    if (k==0 or k==n): return 1
    elif (k==1 or k==n-1): return int(3*n//2)
    else: return T(n-1,k-1) + T(n-1,k)
flatten([[T(n,k) for k in range(n+1)] for n in range(13)]) # G. C. Greubel, Jun 19 2024

T(n, k) = number of paths from (0, 0) to (n-k, k) in the directed graph having vertices (i, j) and edges (i, j)-to-(i+1, j) and (i, j)-to-(i, j+1) for i, j >= 0 and edges (i, j)-to-(i+1, j+1) for i=0, j >= 0 and even and for j=0, i >= 0 and even.
From G. C. Greubel, Jun 19 2024: (Start)
T(n, n-k) = T(n, k)
T(n, 1) = (-1)^n*A084056(n) = A032766(n), n >= 1.
T(n, 2) = A006578(n-1), n >= 2.
T(n, 3) = (1/16)*(4*n^3 - 14*n^2 + 12*n + 15 + (-1)^n) - [n=3] , n >= 3.
Sum_{k=0..n} (-1)^k*T(n, k) = A176742(n) + [n=2].
Sum_{k=0..floor(n/2)} (-1)^k*T(n-k, k) = (1/4)*((-1)^n*(8/sqrt(3)* sin(2*(n+1)*Pi/3) - 2*cos(n*Pi/2) + 1) - 3) + [n<2].
Sum_{k=0..n} k*T(n, k) = (1/6)*n*(17*2^(n-2) - 2 - (1-(-1)^n)) + (1/4)*[n=1]. (End)

A026627 a(n) = A026626(2*n, n).

Original entry on oeis.org

1, 3, 8, 28, 98, 354, 1300, 4834, 18142, 68578, 260720, 995856, 3818644, 14690940, 56677652, 219195454, 849523318, 3298629106, 12829651312, 49973834584, 194917940188, 761178474076, 2975764881352, 11645184195364

Offset: 0

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 0..1000

Cf. A026626, A026628, A026629, A026630, A026631, A026632, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[n le 2 select 2*n-1 else ((357*n^3-2696*n^2+6441*n-4822)*Self(n-1) +2*(2*n-7)*(51*n^2-203*n+188)*Self(n-2))/(2*(n-1)*(51*n^2-305*n+442)): n in [1..41]]; // G. C. Greubel, Jun 19 2024

a[n_]:= a[n]= If[n<2, 2*n+1, ((357*n^3 -1625*n^2 +2120*n -720)*a[n-1] +2*(2*n-5)*(51*n^2 -101*n +36)*a[n-2])/(2*n*(51*n^2-203*n+188))];
Table[a[n], {n,0,40}] (* G. C. Greubel, Jun 19 2024 *)

@CachedFunction
def a(n): # a = A026627
    if n<2: return 2*n+1
    else: return ((357*n^3 -1625*n^2 +2120*n -720)*a(n-1) +2*(2*n-5)*(51*n^2 -101*n +36)*a(n-2))/(2*n*(51*n^2 -203*n +188))
[a(n) for n in range(41)] # G. C. Greubel, Jun 19 2024

a(n) = ( (357*n^3 - 1625*n^2 + 2120*n - 720)*a(n-1) + 2*(2*n-5)*(51*n^2 - 101*n + 36)*a(n-2) )/(2*n*(51*n^2 - 203*n + 188)), for n >= 2, with a(0) = 1, a(1) = 3.

A026628 a(n) = A026626(2*n, n-1).

Original entry on oeis.org

1, 6, 21, 79, 296, 1117, 4237, 16147, 61782, 237208, 913466, 3526826, 13647886, 52920075, 205566205, 799791235, 3116196550, 12157265980, 47485135510, 185671296850, 726703966600, 2846827216330, 11161555459090, 43794648931054

Offset: 1

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 1..1000

Cf. A026626, A026627, A026629, A026630, A026631, A026632, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[n le 2 select 5*n-4 else ((357*n^4-1625*n^3+2157*n^2-841*n+60)*Self(n-1) +2*(2*n-5)*(51*n^3-101*n^2+34*n+6)*Self(n-2))/(2*(n+1)*(51*n^3-254*n^2+389*n-180)): n in [1..41]]; // G. C. Greubel, Jun 19 2024

a[n_]:= a[n]= If[n<3, 5*n-4, ((357*n^4 -1625*n^3 +2157*n^2 -841*n +60 )*a[n-1] +2*(2*n-5)*(51*n^3 -101*n^2 +34*n +6)*a[n-2])/(2*(n+1)*(51*n^3 -254*n^2 +389*n -180))];
Table[a[n], {n,41}]

@CachedFunction
def a(n): # a = A026628
    if n<3: return 5*n-4
    else: return ((357*n^4 -1625*n^3 +2157*n^2 -841*n +60)*a(n-1) +2*(2*n-5)*(51*n^3 -101*n^2 +34*n +6)*a(n-2))/(2*(n+1)*(51*n^3-254*n^2+389*n-180))
[a(n) for n in range(1,41)] # G. C. Greubel, Jun 19 2024

a(n) = ( (357*n^4 - 1625*n^3 + 2157*n^2 - 841*n + 60)*a(n-1) + 2*(2*n-5)*(51*n^3 - 101*n^2 + 34*n + 6)*a(n-2) )/(2*(n+1)*(51*n^3 - 254*n^2 + 389*n - 180)), for n >= 3, with a(1) = 1, a(2) = 6. - G. C. Greubel, Jun 19 2024

a(n) ~ 17 * 2^(2*n-2) / (3*sqrt(Pi*n)). - Vaclav Kotesovec, Aug 09 2025

A026629 a(n) = A026626(2*n, n-2).

Original entry on oeis.org

1, 9, 40, 171, 703, 2839, 11346, 45066, 178330, 704038, 2775590, 10933363, 43048403, 169463371, 667090762, 2626243774, 10340952238, 40727191246, 160443432712, 632240809054, 2492136145078, 9826353817510, 38756552628820

Offset: 2

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 2..1000

Cf. A026626, A026627, A026628, A026630, A026631, A026632, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[n le 2 select 8*n-7 else ((357*n^5 -197*n^4 -465*n^3 -115*n^2 -72*n + 252)*Self(n-1) +2*(2*n-3)*(51*n^4 +52*n^3 -21*n^2 +2*n -12)*Self(n-2))/(2*(n+3)*(51*n^4-152*n^3+129*n^2-4*n-36)): n in [1..41]]; // G. C. Greubel, Jun 20 2024

a[n_]:= a[n]= If[n<4, 8*n-15, ((357*n^5 -1982*n^4 +3893*n^3 -3472*n^2 + 1336*n +120)*a[n-1] + 2*(2*n-5)*(51*n^4 -152*n^3 +129*n^2 -4*n -36)*a[n-2])/(2*(n+2)*(51*n^4 -356*n^3 +891*n^2 -922*n +300))];
Table[a[n], {n,2,41}] (* G. C. Greubel, Jun 20 2024 *)

@CachedFunction
def a(n): # a = A026629
    if n<4: return 8*n-15
    else: return ((357*n^5 -1982*n^4 +3893*n^3 -3472*n^2 +1336*n + 120)*a(n-1) +2*(2*n-5)*(51*n^4 -152*n^3 +129*n^2 -4*n -36)*a(n-2) )/(2*(n+2)*(51*n^4-356*n^3+891*n^2-922*n+300))
[a(n) for n in range(2,41)] # G. C. Greubel, Jun 20 2024

a(n) = ((357*n^5 - 1982*n^4 + 3893*n^3 - 3472*n^2 + 1336*n + 120)*a(n-1) + 2*(2*n-5)*(51*n^4 - 152*n^3 + 129*n^2 - 4*n - 36)*a(n-2))/(2*(n+2)*(51*n^4 - 356*n^3 + 891*n^2 - 922*n + 300)), for n >= 4, with a(2) = 1, a(3) = 9. - G. C. Greubel, Jun 20 2024

A026630 a(n) = A026626(2*n-1, n-1).

Original entry on oeis.org

1, 4, 14, 49, 177, 650, 2417, 9071, 34289, 130360, 497928, 1909322, 7345470, 28338826, 109597727, 424761659, 1649314553, 6414825656, 24986917292, 97458970094, 380589237038, 1487882440676, 5822592097682, 22806739654454

Offset: 1

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 1..1000

Cf. A026626, A026627, A026628, A026629, A026631, A026632, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[1] cat [n le 2 select 2*3^n -2^n else ((357*n^3 -554*n^2 -59*n +132)*Self(n-1) +2*(2*n-3)*(51*n^2 +n -14)*Self(n-2))/(2*(n+1)*(51*n^2-101*n+36)): n in [1..40]]; // G. C. Greubel, Jun 20 2024

a[n_]:= a[n]= If[n<4, 2*3^(n-1) -2^(n-1), ((357*n^3 -1625*n^2 +2120*n - 720)*a[n-1] +2*(2*n-5)*(51*n^2 -101*n +36)*a[n-2])/(2*n*(51*n^2 - 203*n +188))];
Table[a[n], {n,40}] (* G. C. Greubel, Jun 20 2024 *)

@CachedFunction
def a(n): # a = A026630
    if (n<4): return 2*3^(n-1) -2^(n-1)
    else: return ((357*n^3 -1625*n^2 +2120*n - 720)*a(n-1) +2*(2*n-5)*(51*n^2 -101*n +36)*a(n-2))/(2*n*(51*n^2 - 203*n +188))
[a(n) for n in range(1,41)] # G. C. Greubel, Jun 20 2024

a(n) = ((357*n^3 - 1625*n^2 + 2120*n - 720)*a(n-1) +2*(2*n-5)*(51*n^2 - 101*n + 36)*a(n-2))/(2*n*(51*n^2 - 203*n + 188)), with a(1) = 1, a(2) = 4, and a(3) = 14. - G. C. Greubel, Jun 20 2024

A026631 a(n) = A026626(2*n-1, n-2).

Original entry on oeis.org

1, 7, 30, 119, 467, 1820, 7076, 27493, 106848, 415538, 1617504, 6302416, 24581249, 95968478, 375029576, 1466881997, 5742440324, 22498218218, 88212326756, 346114729562, 1358944775654, 5338963361408, 20987909276600

Offset: 2

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 2..1000

Cf. A026626, A026627, A026628, A026629, A026630, A026632, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[1] cat [n le 2 select 23*(n+2)-62 else ((357*n^4 +874*n^3 +495*n^2 - 166*n -192)*Self(n-1) + 2*(2*n-1)*(51*n^3 +154*n^2 +137*n + 42 )*Self(n-2))/(2*(n+3)*(51*n^3 +n^2 -18*n +8)): n in [1..40]]; // G. C. Greubel, Jun 20 2024

a[n_]:= a[n]= If[n<5, 23*n-62, ((357*n^4 -1982*n^3 +3819*n^2 -3082*n + 840)*a[n-1] +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a[n-2] )/(2*(n + 1)*(51*n^3 -305*n^2 +590*n -360))] +17*Boole[n==2];
Table[a[n], {n, 2, 40}] (* G. C. Greubel, Jun 20 2024 *)

@CachedFunction
def a(n): # a = A026631
    if (n==2): return 1
    elif (n<5): return 23*n - 62
    else: return ((357*n^4 -1982*n^3 +3819*n^2 -3082*n +840)*a(n-1) +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a(n-2))/(2*(n+1)*(51*n^3 -305*n^2 +590*n -360))
[a(n) for n in range(2,41)] # G. C. Greubel, Jun 20 2024

a(n) = ((357*n^4 -1982*n^3 +3819*n^2 -3082*n +840)*a(n-1) +2*(2*n-5)*(51*n^3 -152*n^2 +133*n -24)*a(n-2))/(2*(n+1)*(51*n^3 -305*n^2 +590*n -360)), for n >= 5, with a(2) = 1, a(3) = 7, and a(4) = 30. - G. C. Greubel, Jun 20 2024

A026632 a(n) = A026626(n, floor(n/2)).

Original entry on oeis.org

1, 1, 3, 4, 8, 14, 28, 49, 98, 177, 354, 650, 1300, 2417, 4834, 9071, 18142, 34289, 68578, 130360, 260720, 497928, 995856, 1909322, 3818644, 7345470, 14690940, 28338826, 56677652, 109597727, 219195454, 424761659, 849523318

Offset: 0

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 0..1000

Cf. A026626, A026627, A026628, A026629, A026630, A026631, A026633.
Cf. A026634, A026635, A026636, A026961, A026962, A026963, A026964.
Cf. A026965.

[1] cat [n le 4 select Fibonacci(n+2) -(1-(-1)^n)/2 else (4*(867*n^4 - 11934*n^3 +58705*n^2 -123374*n +95280)*Self(n-1) +(6069*n^5 - 91817*n^4 +525005*n^3 -1404375*n^2 +1742414*n -803760)*Self(n-2) +2*(867*n^4 -11934*n^3 +58705*n^2 -123374*n +95280)*Self(n-3) +4*(n-5)*(867*n^4 -8534*n^3 +28921*n^2 -39246*n +17712)*Self(n-4))/(2*(n+1)*(867*n^4 -12002*n^3 +59725*n^2 -126158*n +95280)): n in [1..40]]; // G. C. Greubel, Jun 20 2024

T[n_, k_]:= T[n, k]= If[k==0 || k==n, 1, If[k==1 || k==n-1, (6*n-1 + (-1)^n)/4, T[n-1,k-1] +T[n-1,k]]];
Table[T[n, Floor[n/2]], {n,0,40}] (* G. C. Greubel, Jun 20 2024 *)

@CachedFunction
def T(n, k): # T = A026626
    if (k==0 or k==n): return 1
    elif (k==1 or k==n-1): return int(3*n//2)
    else: return T(n-1, k-1) + T(n-1, k)
[T(n,int(n//2)) for n in range(41)] # G. C. Greubel, Jun 20 2024

a(n) = (4*(867*n^4 -11934*n^3 +58705*n^2 -123374*n +95280)*a(n-1) +(6069*n^5 -91817*n^4 + 525005*n^3 -1404375*n^2 +1742414*n -803760 )*a(n-2) +2*(867*n^4 - 11934*n^3 +58705*n^2 -123374*n +95280)*a(n-3) + 4*(n-5)*(867*n^4 - 8534*n^3 +28921*n^2 -39246*n +17712)*a(n-4))/(2*(n+1)*(867*n^4 - 12002*n^3 +59725*n^2 -126158*n +95280)), for n >= 6, with a(0) = a(1) = 1, a(2) = 3, a(3) = 4, and a(4) = 8. - G. C. Greubel, Jun 20 2024

A026633 a(n) = Sum_{k=0..n} A026626(n, k).

Original entry on oeis.org

1, 2, 5, 10, 22, 44, 90, 180, 362, 724, 1450, 2900, 5802, 11604, 23210, 46420, 92842, 185684, 371370, 742740, 1485482, 2970964, 5941930, 11883860, 23767722, 47535444, 95070890, 190141780, 380283562, 760567124, 1521134250

Offset: 0

Clark Kimberling

nonn

G. C. Greubel, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (2,1,-2).

Row sums of A026626.
Cf. A026627, A026628, A026629, A026630, A026631, A026632, A026634.
Cf. A026635, A026636, A026961, A026962, A026963, A026964, A026965.

[n le 1 select n+1 else (17*2^(n-2) +(-1)^n)/3 -1: n in [0..40]]; // G. C. Greubel, Jun 21 2024

Table[(17*2^(n-2) +(-1)^n)/3 -1 +Boole[n==0]/4 +Boole[n==1]/2, {n, 0, 40}] (* G. C. Greubel, Jun 21 2024 *)

[(17*2^(n-2) +(-1)^n)/3 -1 +int(n==0)/4 +int(n==1)/2 for n in range(41)] # G. C. Greubel, Jun 21 2024

G.f.: (1+x^4)/((1-2*x)*(1-x^2)). - Ralf Stephan, Apr 30 2004
a(n) = (1/3)*(17*2^(n-2) + (-1)^n) - 1, n>=2. - R. J. Mathar, May 22 2013
a(n) = a(n-1) + 2*a(n-2) + 2. - Greg Dresden, Feb 22 2020
E.g.f.: (3 + 6*x - 8*cosh(x) + 17*cosh(2*x) - 16*sinh(x) + 17*sinh(2*x))/12. - Stefano Spezia, Feb 22 2020

cp's OEIS Frontend

A026635 a(n) = Sum_{i=0..n} Sum_{j=0..n} A026626(i,j).

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A026636 a(n) = Sum_{k=0..floor(n/2)} A026626(n-k, k).

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A026626 Triangular array T read by rows: T(n,0) = T(n,n) = 1 for n >= 0, T(n,1) = T(n,n-1) = floor(3*n/2) for n >= 1, T(n,k) = T(n-1,k-1) + T(n-1,k) for 2 <= k <= n-2, n >= 4.

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A026627 a(n) = A026626(2*n, n).

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A026628 a(n) = A026626(2*n, n-1).

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A026629 a(n) = A026626(2*n, n-2).

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A026630 a(n) = A026626(2*n-1, n-1).

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