We describe several techniques for constructing -dimensional Hadamard matrices from -dimensional Hadamard matrices, and note that they may be applied to any perfect binary array , thus optimally improving a result of Yang. We introduce cocyclic perfect binary arrays, whose energy is not restricted to being a perfect square. These include
all of Jedwab’s generalized perfect binary arrays. There are many more cocyclic than . We resolve a potential ambiguity inherent in the “weak difference set” construction of -dimensional Hadamard matrices from cocyclic and show it
is a relative difference set construction.