OFFSET
1,1
COMMENTS
Twice A025052, see there for further comments.
FORMULA
a(n) = 2 * A025052(n).
Surface area = 2*(n1*n2 + n1*n3 + n2*n3).
EXAMPLE
A 1 X 1 X 1 block has surface area 6. A 1 X 1 X 2 block has surface area 10. No n1 X n2 X n3 block of intermediate size exists, so there is no way to create an n1 X n2 X n3 block with surface area 8.
PROG
(Python)
from sympy import integer_nthroot
def aupto(lim):
e, r, lim2 = set(range(2, lim+1, 2)), set(), integer_nthroot(lim//2, 2)[0]
for n1 in range(1, lim2):
for n2 in range(n1, lim2):
for n3 in range(n2, lim+1):
r.add(2*(n1*n2 + n1*n3 + n2*n3))
return sorted(e - r)
print(aupto(1000)) # Michael S. Branicky, Feb 04 2021
CROSSREFS
KEYWORD
nonn,fini
AUTHOR
Andreas Boe, Sep 05 2014
STATUS
approved