OFFSET
0,3
COMMENTS
For k = 2,3,4,... the difference a(n+k) - a(n) is divisible by k.
LINKS
Ludovic Schwob, Table of n, a(n) for n = 0..199
FORMULA
a(n) = Sum_{k=0..n} 7^(n-k)*(n!/k!)*binomial(n-1, k-1).
Recurrence: a(n) = (14*n-13)*a(n-1) - 49*(n-2)*(n-1)*a(n-2).
a(n) ~ n! * exp(2*sqrt(n/7) - 1/14) * 7^(n - 1/4) / (2 * sqrt(Pi) * n^(3/4)). - Vaclav Kotesovec, Nov 21 2018
MAPLE
seq(coeff(series(factorial(n)*exp(x/(1-7*x)), x, n+1), x, n), n = 0 .. 17); # Muniru A Asiru, Nov 24 2018
MATHEMATICA
a[n_] := Sum[7^(n - k)*n!/k!*Binomial[n - 1, k - 1], {k, 0, n}]; Array[a, 20, 0] (* or *) a[0] = a[1] = 1; a[n_] := a[n] = (14n - 13)*a[n - 1] - 49(n - 2)(n - 1)*a[n - 2]; Array[a, 20, 0] (* Amiram Eldar, Nov 19 2018 *)
PROG
(PARI) my(x='x + O('x^20)); Vec(serlaplace(exp(x/(1-7*x)))) \\ Michel Marcus, Nov 25 2018
(Magma) [1] cat [&+[7^(n-k)*Factorial(n)/Factorial(k)*Binomial(n-1, k-1): k in [0..n]]: n in [1.. 18]]; // Vincenzo Librandi, Dec 08 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Ludovic Schwob, Nov 19 2018
STATUS
approved