Problem 265Published on Saturday, 21st November 2009, 09:00 am; Solved by 2223
2N binary digits can be placed in a circle so that all the N-digit clockwise subsequences are distinct.
For N=3, two such circular arrangements are possible, ignoring rotations:
For the first arrangement, the 3-digit subsequences, in clockwise order, are:
000, 001, 010, 101, 011, 111, 110 and 100.
Each circular arrangement can be encoded as a number by concatenating the binary digits starting with the subsequence of all zeros as the most significant bits and proceeding clockwise. The two arrangements for N=3 are thus represented as 23 and 29:
Calling S(N) the sum of the unique numeric representations, we can see that S(3) = 23 + 29 = 52.