|
|
A366971
|
|
a(n) = Sum_{k=3..n} binomial(k,3) * floor(n/k).
|
|
6
|
|
|
0, 0, 1, 5, 15, 36, 71, 131, 216, 346, 511, 756, 1042, 1441, 1907, 2527, 3207, 4128, 5097, 6371, 7737, 9442, 11213, 13538, 15848, 18734, 21744, 25423, 29077, 33743, 38238, 43818, 49440, 56104, 62694, 70979, 78749, 88154, 97580, 108790, 119450, 132680, 145021, 159974
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,4
|
|
LINKS
|
|
|
FORMULA
|
G.f.: 1/(1-x) * Sum_{k>=1} x^(3*k)/(1-x^k)^4 = 1/(1-x) * Sum_{k>=3} binomial(k,3) * x^k/(1-x^k).
|
|
PROG
|
(PARI) a(n) = sum(k=3, n, binomial(k, 3)*(n\k));
(Python)
from math import isqrt, comb
def A366971(n): return -comb((s:=isqrt(n))+1, 4)*(s+1)+sum(comb((q:=n//w)+1, 4)+(q+1)*comb(w, 3) for w in range(1, s+1)) # Chai Wah Wu, Oct 30 2023
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|