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 A246850 Even numbers which cannot be represented by the surface area of an n1 X n2 X n3 block. 0
 2, 4, 8, 12, 20, 36, 44, 60, 84, 116, 140, 156, 204, 260, 380, 420, 660, 924 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Twice A025052, see there for further comments. LINKS 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 Cf. A025052. Sequence in context: A100684 A131770 A322419 * A294066 A163489 A095352 Adjacent sequences:  A246847 A246848 A246849 * A246851 A246852 A246853 KEYWORD nonn,fini AUTHOR Andreas Boe, Sep 05 2014 STATUS approved

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Last modified July 25 03:53 EDT 2021. Contains 346283 sequences. (Running on oeis4.)