OFFSET
0,2
COMMENTS
Also the number of compositions of n where each part i is marked with a word of length i over a septenary alphabet whose letters appear in alphabetical order.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..975
Index entries for linear recurrences with constant coefficients, signature (14,-42,70,-70,42,-14,2).
FORMULA
G.f.: (1-x)^7/(2*(1-x)^7-1).
a(n) = A261780(n,7).
a(n) = Sum_{k>=0} (1/2)^(k+1) * binomial(n-1+7*k,n). - Seiichi Manyama, Aug 06 2024
MAPLE
a:= proc(n) option remember; `if`(n=0, 1,
add(a(n-j)*binomial(j+6, 6), j=1..n))
end:
seq(a(n), n=0..20);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Sep 01 2015
STATUS
approved