If $p^a\mathrm{\mid }\mathrm{\mid }n$ where $n>4$ is the order of a Circulant Hadamard matrix, then the order of $p$ modulo $n\mathrm{/}{p}^a$ is odd

We proved recently (see \cite{lhgarasu}) the result on the title for odd prime divisors of such an $n\mathrm{.}$ The result implies for many $n^{\mathrm{\prime }}s$, more precisely, for an infinity of $n$’s with an arbitrary fixed number of prime divisors, the inexistence of circulant Hadamard matrices, and the inexistence of Barker sequences of length $n>13$. The proof used a result of Arasu. It turns out that there is another, shorter proof, of the more general result that includes the prime $p=2.$ This new proof is based on a result of Brock (see \cite[Theorem 3.1]{brock}), and besides that, requires just the definition of the Fourier transform. I noticed Brock’s result in a preprint (see \cite{winterhofetal}) of Winterhof et al. where it is used to study the inexistence of related Butson-Hadamard matrices.