A026560 a(n) = T(2*n, n-2), where T is given by A026552.

1, 4, 18, 74, 311, 1296, 5432, 22796, 95958, 404812, 1711600, 7250970, 30772989, 130810512, 556867224, 2373764416, 10130935783, 43285462884, 185129287262, 792525473552, 3395664830670, 14560682746632, 62482560679368, 268307898599664, 1152883194581155, 4956738399534376, 21323028570642414, 91775945084805898

Offset

2,2

Links

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

Formula

a(n) = A026552(2*n, n-2).

Mathematica

T[n_, k_]:= T[n, k]= If[k==0 || k==2*n, 1, If[k==1 || k==2*n-1, Floor[(n+2)/2], If[EvenQ[n], T[n-1, k-2] + T[n-1, k] + T[n-1, k-1], T[n-1, k-2] + T[n-1, k]]]]; (* T=A026552 *)
a[n_]:= a[n]= Block[{$RecursionLimit = Infinity}, T[2*n, n-2]];
Table[a[n], {n, 2, 40}] (* G. C. Greubel, Dec 18 2021 *)

Prog

(Sage)
@CachedFunction
def T(n, k): # T = A026552
    if (k==0 or k==2*n): return 1
    elif (k==1 or k==2*n-1): return (n+2)//2
    elif (n%2==0): return T(n-1, k) + T(n-1, k-1) + T(n-1, k-2)
    else: return T(n-1, k) + T(n-1, k-2)
[T(2*n, n-2) for n in (2..40)] # G. C. Greubel, Dec 18 2021

Crossrefs

Cf. A026552, A026553, A026554, A026555, A026556, A026557, A026558, A026559, A026563, A026566, A026567, A027272, A027273, A027274, A027275, A027276.

Sequence in context: A307323 A218059 A069008 * A180140 A245127 A037674

Adjacent sequences: A026557 A026558 A026559 * A026561 A026562 A026563

Keyword

nonn

Author

Clark Kimberling

Extensions

Terms a(20) onward from G. C. Greubel, Dec 18 2021

Status

approved