Power of three

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In mathematics, a power of three is a number of the form 3n where n is an integer – that is, the result of exponentiation with number three as the base and integer n as the exponent.



The powers of three give the place values in the ternary numeral system. [1]

In graph theory, powers of three appear in the Moon–Moser bound 3n/3 on the number of maximal independent sets of an n-vertex graph, [2] and in the time analysis of the Bron–Kerbosch algorithm for finding these sets. [3] Several important strongly regular graphs also have a number of vertices that is a power of three, including the Brouwer–Haemers graph (81 vertices), Berlekamp–van Lint–Seidel graph (243 vertices), and Games graph (729 vertices). [4]

In enumerative combinatorics, there are 3n signed subsets of a set of n elements. In polyhedral combinatorics, the hypercube and all other Hanner polytopes have a number of faces (not counting the empty set as a face) that is a power of three. For example, a 2-cube, or square, has 4 vertices, 4 edges and 1 face, and 4 + 4 + 1 = 32. Kalai's 3d conjecture states that this is the minimum possible number of faces for a centrally symmetric polytope. [5]

In recreational mathematics and fractal geometry, inverse power-of-three lengths occur in the constructions leading to the Koch snowflake, [6] Cantor set, [7] Sierpinski carpet and Menger sponge, in the number of elements in the construction steps for a Sierpinski triangle, and in many formulas related to these sets. There are 3n possible states in an n-disk Tower of Hanoi puzzle or vertices in its associated Hanoi graph. [8] In a balance puzzle with w weighing steps, there are 3w possible outcomes (sequences where the scale tilts left or right or stays balanced); powers of three often arise in the solutions to these puzzles, and it has been suggested that (for similar reasons) the powers of three would make an ideal system of coins. [9]

In number theory, all powers of three are perfect totient numbers. [10] The sums of distinct powers of three form a Stanley sequence, the lexicographically smallest sequence that does not contain an arithmetic progression of three elements. [11] A conjecture of Paul Erdős states that this sequence contains no powers of two other than 1, 4, and 256. [12]

Graham's number, an enormous number arising from a proof in Ramsey theory, is (in the version popularized by Martin Gardner) a power of three. However, the actual publication of the proof by Ronald Graham used a different number. [13]

The 0th to 63rd powers of three

(sequence A000244 in the OEIS )

30= 1 316=43046721332=1853020188851841348=79766443076872509863361
31= 3 317=129140163333=5559060566555523349=239299329230617529590083
32= 9 318=387,420,489334=16677181699666569350=717897987691852588770249
33= 27 319=1162261467335=50031545098999707351=2153693963075557766310747
34= 81 320=3486784401336=150094635296999121352=6461081889226673298932241
35= 243 321=10460353203337=450283905890997363353=19383245667680019896796723

See also

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Berlekamp–Van Lint–Seidel graph

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  4. For the Brouwer–Haemers and Games graphs, see Bondarenko, Andriy V.; Radchenko, Danylo V. (2013), "On a family of strongly regular graphs with ", Journal of Combinatorial Theory , Series B, 103 (4): 521–531, arXiv: 1201.0383 , doi: 10.1016/j.jctb.2013.05.005 , MR   3071380 . For the Berlekamp–van Lint–Seidel and Games graphs, see van Lint, J. H.; Brouwer, A. E. (1984), "Strongly regular graphs and partial geometries" (PDF), in Jackson, David M.; Vanstone, Scott A. (eds.), Enumeration and Design: Papers from the conference on combinatorics held at the University of Waterloo, Waterloo, Ont., June 14–July 2, 1982, London: Academic Press, pp. 85–122, MR   0782310
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  8. Hinz, Andreas M.; Klavžar, Sandi; Milutinović, Uroš; Petr, Ciril (2013), "2.3 Hanoi graphs", The tower of Hanoi—myths and maths, Basel: Birkhäuser, pp. 120–134, doi:10.1007/978-3-0348-0237-6, ISBN   978-3-0348-0236-9, MR   3026271
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  13. Gardner, Martin (November 1977), "In which joining sets of points leads into diverse (and diverting) paths", Scientific American, 237 (5): 18–28, doi:10.1038/scientificamerican1177-18