Answering a question of Pott on almost perfect sequences (Q1591650)

Binary sequences with an almost perfect autocorrelation spectrum are \(\pm 1\) sequences where all except one out-of-phase autocorrelation coefficients are \(0\). Large classes of these sequences are equivalent to cyclic relative difference sets with parameters \((n+1, 2, n, (n-1)/2)\). It is conjectured that these difference sets exist if and only if \(n\) is a prime power. For \(n=425\) it was previously unknown whether such a difference set can exist. Using multipliers, the authors present a nonexistence proof for these difference sets and therefore for certain almost perfect sequences.