# Group Theory in Rubik's Cube. Pritish Kamath special thanks to Nishant Totla

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1 Group Theory in Rubik's Cube Pritish Kamath special thanks to Nishant Totla

2 Overview Notations Counting Rubik's cube configurations... Quick review of Group Theory. The Rubik's Cube Group. More Group Theory and representation of Rubik's Cube Configurations Counting valid Rubik's Cube Configurations. Optimal solution of the Rubik's Cube

3 Notations The Rubik s cube is composed of 27 small cubes, which are typically called cubies. 26 of these cubies are visible (infact, the 27th cubie doesn t actually exist). Types of cubies : 8 corner cubies 12 edge cubies 6 center cubies Cubicles describe the space in which the Cubies lie.

4 Number of possible Cube Configurations 8! = number of ways of arranging the corner cubies 3 8 = number of ways of orienting the corner cubies 12! = number of ways of arranging the edge cubies 2 12 = number of ways of orienting the edge cubies Total number of configurations = !.12! = 519,024,039,293,878,272,000 = ~519 x These configurations that can be obtained by breaking the cube and fixing it again. The center cubies remain where they are, even after breaking open the cube.

5 Number of possible Cube Configurations These configurations are theoretically possible, but that doesn t mean that these configurations could really occur. A configuration of the Rubik s cube is said to be valid if it can be achieved by a series of moves from the starting configuration. It turns out that some of the theoretically possible configurations, we have just counted, are actually not valid. Therefore, we have two goals: Demonstrate that some configurations are not valid. Find a set of moves that can take us from any valid configuration back to the start configuration.

6 Group Theory Quick Review A group (G, ) consists of a set G and an operation such that: if a, b G, then a b G Closure if a,b,c There exists e G, then a*(b*c) = (a*b)*c Associativity G s.t. g*e = e*g = g Existence of Identity For all g G, there exists h G, s.t. g*h = h*g = e Existence of Inverse

7 The Rubik's Cube Group The set of all possible moves on a Rubik's cube form the Rubik's Cube Group. The operation * is the simple composition rule. Two moves are considered same, if the final configuration after the moves are the same. Thus, this set is, in fact, similar to the set of all valid configurations, as discussed earlier.

8 Generators of a group Let (G,*) be a group and S be a subset of G. We say that S generates G or that S is a set of generators of G if every element of G can be written as a finite product (under the group operation) of elements of S and their inverses G = <S> In case of a finite group G, S generates G iff every element is expressible as finite product of elements of S (that is, inverses of S are not required)

9 Generators of the Rubik's Cube Group S = {U, D, L, R, F, B} G U : clockwise rotation of top face D : clockwise rotation of bottom face L : clockwise rotation of left face R : clockwise rotation of right face F : clockwise rotation of front face B : clockwise rotation of back face S is the generator of the group G

10 The Symmetric Group, S n The symmetric group on n letters is the set of bijections from {1, 2,..., n} to {1, 2,..., n}, with the operation of composition. This group is represented by as S n Thus, given any configuration of the Rubik's Cube, the location of the corner cubies can be represented by an element in S 8 the location of the edge cubies can be represented by an element in S 12

11 Representing Rubik's Cube Configurations A configuration of the Rubik s cube is completely determined by four pieces of data: the positions of the corner cubies the positions of the edge cubies the orientations of the corner cubies the orientations of the edge cubies Positions of corner cubies and edge cubies, are represented by elements of S 8 and S 12 respectively Claim : All elements of S 8 and S 12 are reachable.

12 Representing orientations of Cubies Each corner cubie has 3 possible orientations, and we will number these orientations 0, 1, and 2. Figures below indicate a possible assignment of these numbers to the cube orientations.

13 Representing orientations of Cubies If the Rubik s cube is in any configuration, we will describe the orientations of the corner cubies as : For any i between 1 and 8, find the cubicle face labeled i. Let x i be the number of the cubie face living in this cubicle face. Let x be the ordered 8-tuple ( x 1, x 2,..., x 7, x 8 ) x i is the number of clockwise twists the cubie 'i' is away from having its 0 face in the numbered face of the cubicle. Similarly the orientation of the 12 edge cubies can be represented by a 12-tuple ( y 1, y 2,, y 11, y 12 ) where each y i, is either 0 or 1.

14 Representing Rubik's Cube Configurations Thus, any Rubik's Cube Configuration can be represented by a 4-tuple (σ, τ, x, y) : σ τ S 8 Permutations of corner cubies S 12 Permutations of edge cubies x = (x 1, x 2,..., x 7, x 8 ) orientation of corner cubies y = (y 1, y 2,, y 11, y 12 ) orientation of edge cubies

15 Valid Cube Configurations Theorem : A configuration (σ, τ, x, y) is valid iff sgn σ = sgn τ, Σ x i 0 (mod 3), and Σ y i 0 (mod 2) The proof is not very difficult, but has a large amount of detail, which we'll skip here. By this theorem, the total number of valid configurations is reduced by a factor of 2x3x2 = 12. Thus the total number of valid configurations = 43,252,003,274,489,856,000 = ~43 x 10 18

16 Optimal Solution of the Cube and the Cayley Graph A Cayley graph, or the Cayley colour graph, is a graph that encodes the structure of a discrete group. Suppose, G is a group, with a generator S, Two vertices a and b are adjacent in the Cayley graph, iff there exists s S, such that either (a = b*s) or (b = a*s). Claim : Diameter of the Cayley graph generated by the Rubik's cube is 22. which means, any cube configuration can be solved in atmost 22 steps!!

17 Thank you

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