Friendly number: Difference between revisions

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In [[mathematics]], the '''cake number''', denoted by ''C<sub>n</sub>'', is the maximum number of regions into which a 3-dimensional cube can be partitioned by exactly ''n'' [[plane (geometry)|plane]]s. The cake number is so-called because one may imagine each partition of the cube by a plane as a slice made by a knife through a cube-shaped cake.
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The values of ''C<sub>n</sub>'' for increasing {{nowrap|1=''n'' ≥ 0}} are given by {{nowrap|1=1, 2, 4, 8, 15, 26, 42, 64, 93, &hellip;}}<ref>{{citeweb|url=http://oeis.org/A000125|author=The On-Line Encyclopedia of Integer Sequences|title=A000125: Cake Numbers|accessdate=August 19, 2010}}</ref>
 
The cake numbers are the 3-dimensional analogue of the 2-dimensional [[lazy caterer's sequence]]; the difference between successive cake numbers also gives the lazy caterer's sequence.
 
== General formula ==
 
If ''n''! denotes the [[factorial]], and we denote the [[binomial coefficient]]s by
:<math>  {n \choose k} = \frac{n!}{k! \, (n-k)!} , </math>
and we assume that ''n'' planes are available to partition the cube, then the number is:<ref>{{citeweb|url=http://mathworld.wolfram.com/SpaceDivisionbyPlanes.html|title=Space Division by Planes|author=Eric Weisstein|location=MathWorld − A Wolfram Web Resource|accessdate=August 19, 2010}}</ref>
:<math>
C_n = {n \choose 3} + {n \choose 2} + {n \choose 1} + {n \choose 0} = \frac{1}{6}(n^3 + 5n + 6).  </math>
 
== References ==
{{Reflist}}
 
[[Category:Mathematical optimization]]
 
{{combin-stub}}

Latest revision as of 16:07, 19 December 2014

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