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 A090767 Numbers of the form 3*x*y*z + 2(x*y + y*z + z*x) + (x + y + z) for x, y, z positive integers. 4
 12, 20, 28, 33, 36, 44, 46, 52, 54, 59, 60, 64, 68, 72, 75, 76, 82, 84, 85, 92, 96, 98, 100, 104, 105, 108, 111, 116, 117, 118, 124, 128, 132, 133, 136, 137, 138, 140, 144, 148, 150, 151, 154, 156, 159, 162, 163, 164, 170, 172, 174, 176, 180, 184, 188, 189, 190 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is the set of numbers which count the unit sticks or unit segments needed to construct a three-dimensional cubic lattice made up from unit cubes. This generalizes the two-dimensional version which is A047845 (numbers of the form 2*x*y + x + y for x and y positive integers) and is also the numbers of sticks needed to construct a rectangular lattice of unit squares. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE a(1) = 12 because there are 12 edges to a cube. MAPLE SeqGen1 := proc(n, N) local a, b, c, F, V, v; # n specifies the search space; N specifies the maximal number to appear in the initial segment of the sequence F := 3*x*y*z + 2*(x*y+y*z+z*x)+x+y+z; V := {}; for a from 1 to n do for b from 1 to n do for c from b to n do v := subs(x=a, y=b, F); if v < N then V := V union {v}; fi; od; od; sort(V) end: # alternative: N:= 1000: # to get all terms <= N S:= {seq(seq(seq(3*x*y*z + 2*(x*y+y*z+z*x)+(x+y+z), z = 1 .. min(y, (-2*x*y+N-x-y)/(3*x*y+2*x+2*y+1))), y = 1 .. min(x, (N-3*x-1)/(5*x+3))), x = 1 .. (N-4)/8)}: sort(convert(S, list)); # Robert Israel, Feb 18 2016 MATHEMATICA M = 1000; S = Table[3 x y z + 2(x y + y z + z x) + (x + y + z), {x, 1, (M - 4)/8}, {y, 1, Min[x, (M - 3 x - 1)/(5 x + 3)]}, {z, 1, Min[y, (-2 x y + M - x - y)/(3 x y + 2 x + 2 y + 1)]}] // Flatten // Union (* Jean-François Alcover, Apr 11 2019, after Robert Israel *) CROSSREFS Cf. A047845. Sequence in context: A035511 A095035 A108027 * A117227 A110187 A096156 Adjacent sequences:  A090764 A090765 A090766 * A090768 A090769 A090770 KEYWORD nonn AUTHOR John H. Mason, Feb 02 2004 EXTENSIONS More terms from Ray Chandler, Feb 04 2004 STATUS approved

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Last modified April 6 18:49 EDT 2020. Contains 333286 sequences. (Running on oeis4.)