How many ways there are of colouring the 11 beads of this necklace either red or green if reflections and rotations are considered to be identical?
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The Polya Enumeration Theorem tells us that the number of ways N of colouring the beads can be found by obtaining the cycle index of the dihedral group D 1 1 ; note that Z ( D n ) = 2 1 Z ( C n ) + ⎩ ⎪ ⎨ ⎪ ⎧ 2 1 a 1 a 2 2 n − 1 4 1 ( a 2 2 n + a 1 2 a 2 2 n − 2 ) if n ≡ 1 ( mod 2 ) if n ≡ 0 ( mod 2 ) , where C n is the cyclic group, and Z ( C n ) = n 1 k ∣ n ∑ ϕ ( k ) a k k n . Evaluating this sum at n = 1 1 and substituting a i = r i + g i (to account for the two colours) yields the generating function Z ( D 1 1 ) [ r + g ] = r 1 1 + g 1 1 + r 1 0 g + r g 1 0 + 5 r 9 g 2 + 5 r 2 g 9 + 1 0 r 8 g 3 + 1 0 r 3 g 8 + 2 0 r 7 g 4 + 2 0 r 4 g 7 + 2 6 r 6 g 5 + 2 6 r 5 g 6 , and thus N = 1 + 1 + 1 + 1 + 5 + 5 + 1 0 + 1 0 + 2 0 + 2 0 + 2 6 + 2 6 = 1 2 6 .