A026642 a(n) = A026637(2*n-1, n-2).

1, 7, 28, 112, 439, 1711, 6652, 25846, 100450, 390670, 1520764, 5925718, 23112931, 90239407, 352654084, 1379410438, 5400188206, 21157958962, 82959736504, 325514137048, 1278093308806, 5021436970822, 19740128055928

Offset

2,2

Links

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

Formula

a(n) = ( (7*n^2 - 11*n + 6)*a(n-1) + 2*(n-1)*(2*n-1)*a(n-2) )/(2*(n-2)*(n+1)), n >= 4. - G. C. Greubel, Jul 01 2024

Mathematica

a[n_]:= a[n]= If[n<4, 7^(n-2), ((7*n^2-11*n+6)*a[n-1] + 2*(n-1)*(2*n- 1)*a[n-2])/(2*(n-2)*(n+1))];
Table[a[n], {n, 2, 40}] (* G. C. Greubel, Jul 01 2024 *)

Prog

(Magma)
[n le 2 select 7^(n-1) else ((7*n^2+3*n+2)*Self(n-1) + 2*n*(2*n+1)*Self(n-2))/(2*(n-1)*(n+2)): n in [1..40]]; // G. C. Greubel, Jul 01 2024
(SageMath)
@CachedFunction
def a(n): # a = A026642
    if n<4: return 7^(n-2)
    else: return ((7*n^2-11*n+6)*a(n-1) + 2*(n-1)*(2*n-1)*a(n-2))/(2*(n-2)*(n+1))
[a(n) for n in range(2, 41)] # G. C. Greubel, Jul 01 2024

Crossrefs

Cf. A026637, A026638, A026639, A026640, A026641, A026643, A026644.

Cf. A026645, A026646, A026647, A026966, A026967, A026968, A026969.

Cf. A026970.

Sequence in context: A200762 A243150 A344206 * A200467 A002042 A200666

Adjacent sequences: A026639 A026640 A026641 * A026643 A026644 A026645

Keyword

nonn

Author

Clark Kimberling

Status

approved