Purpose: In analogy with the ordinary and the periodic Golay pairs, we introduce also the negaperiodic Golay pairs.
(They occurred first, under a different name, in a paper of Ito.) Methods: We investigate the construction of Hadamard (and
weighing) matrices from two negacyclic blocks (2N-type). The Hadamard matrices of 2N-type are equivalent to negaperiodic
Golay pairs. Results: If a Hadamard matrix is also a Toeplitz matrix, we show that it must be either cyclic or negacyclic. We
show that the Turyn multiplication of Golay pairs extends to a more general multiplication: one can multiply Golay pairs of
length g and negaperiodic Golay pairs of length v to obtain negaperiodic Golay pairs of length gv. We show that the Ito’s
conjecture about Hadamard matrices is equivalent to the conjecture that negaperiodic Golay pairs exist for all even lengths.
Practical relevance: Hadamard matrices have direct practical applications to the problems of noise-immune coding and
compression and masking of video information.