OFFSET
0,2
FORMULA
a(n) ~ c * 2^(n*(n+2)) * exp(Pi*n*(n+2)/2 - 3*n^2) * n^(2*n^2 - 2 - Pi/4), where c = 0.4952828896469310726828820344381813905230827930914109676983577406850360879...
MATHEMATICA
Table[Product[i*(i+1)+j*(j+1), {i, 1, n}, {j, 1, n}], {n, 0, 8}]
PROG
(Python)
from math import prod, factorial
def A306728(n): return (prod(i*(i+1)+j*(j+1) for i in range(1, n) for j in range(i+1, n+1))*factorial(n))**2*(n+1)<<n # Chai Wah Wu, Nov 22 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, Mar 06 2019
STATUS
approved