OFFSET
0,1
LINKS
C. Mackeprang & K. Myers, Coloring Graphs on Sponges: Problem 11208, Amer. Math. Monthly 114 (November 2007), p. 842.
FORMULA
a(n) = floor((7 + sqrt(1 + 48*(21*20^n + 38*8^n - 59)/133))/2).
EXAMPLE
a(0)=4 because a cube requires at most 4 colors. a(1)=11 because a cube with holes drilled through the faces meeting in the center requires at most 11 colors.
MATHEMATICA
Table[Floor[(7+Sqrt[1+48*(21*20^n+38*8^n-59)/133])/2], {n, 0, 30}] (* Harvey P. Dale, Mar 07 2012 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Marc LeBrun, Dec 05 2007
STATUS
approved