This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A246850 #22 Feb 05 2021 00:21:48 %S A246850 2,4,8,12,20,36,44,60,84,116,140,156,204,260,380,420,660,924 %N A246850 Even numbers which cannot be represented by the surface area of an n1 X n2 X n3 block. %C A246850 Twice A025052, see there for further comments. %F A246850 a(n) = 2 * A025052(n). %F A246850 Surface area = 2*(n1*n2 + n1*n3 + n2*n3). %e A246850 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. %o A246850 (Python) %o A246850 from sympy import integer_nthroot %o A246850 def aupto(lim): %o A246850 e, r, lim2 = set(range(2, lim+1, 2)), set(), integer_nthroot(lim//2, 2)[0] %o A246850 for n1 in range(1, lim2): %o A246850 for n2 in range(n1, lim2): %o A246850 for n3 in range(n2, lim+1): %o A246850 r.add(2*(n1*n2 + n1*n3 + n2*n3)) %o A246850 return sorted(e - r) %o A246850 print(aupto(1000)) # _Michael S. Branicky_, Feb 04 2021 %Y A246850 Cf. A025052. %K A246850 nonn,fini %O A246850 1,1 %A A246850 _Andreas Boe_, Sep 05 2014