http://mathworld.i2p/BarkerCode.html
A list of known Barker codes up to reversal of digits and negation is given below. length code 2 , 3 4 , 5 7 11 13 The number of candidate codes of length is therefore equal to the number of -bead black-white reversible strings 1, 2, 3, 6, 10, 20, 36, 72, ...