OFFSET
1,4
FORMULA
G.f.: 1/(1-x) * Sum_{k>=1} x^(3*k)/(1-x^k)^2 = 1/(1-x) * Sum_{k>=3} (k-2) * x^k/(1-x^k).
PROG
(PARI) a(n) = sum(k=3, n, (k-2)*(n\k));
(Python)
from math import isqrt
def A366969(n): return n+(-(s:=isqrt(n))*(s*(s-2)-7)+sum(((q:=n//w)+1)*(q+(w<<1)-8) for w in range(1, s+1))>>1) # Chai Wah Wu, Oct 30 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Oct 30 2023
STATUS
approved