A Local Valuation Criterion for Quadratic-Permutation Interleaved Zadoff--Chu Sequences

Berggren and Popovi\'c introduced quadratic-permutation-polynomial interleaved Zadoff--Chu sequences and, from exhaustive data, conjectured that all normalized QPP-interleaved Zadoff--Chu sequences are inequivalent to ordinary Zadoff--Chu sequences precisely for prime-power lengths $N=p^n$ with $p>3$ and $n>1$. We give an exact local arithmetic criterion. For a normalized QPP $\pi_{a,b}(k)=ak^2+bk\pmod N$, the interleaved sequence is equivalent, under the standard five CAZAC-preserving operations, to a Zadoff--Chu sequence if and only if, for every prime power $p^\alpha\Vert N$, the valuation of $a$ satisfies $$\nu_p(a)\ge \begin{cases} 0, & p=2,\ \alpha=1,\\ \alpha-1, & p=2,\ \alpha\ge2,\\ \alpha-1, & p=3,\\ \alpha, & p>3. \end{cases}$$ The proof is based on a third finite-difference invariant of the lifted Zadoff--Chu phase, namely $$\Delta^3\bigl((ak^2+bk+\varepsilon_N+2q)(ak^2+bk)\bigr) =12a(2ak+3a+b).$$ As a consequence, the conjectured prime-power boundary is not correct: the exact non-vacuous condition for all nonzero normalized QPPs to be inequivalent to Zadoff--Chu sequences is that $N$ is odd, $9\nmid N$, and $p^2\mid N$ for at least one prime $p\ge5$. In particular, $N=75=3\cdot5^2$ is the smallest non-prime-power counterexample to the conjectured ``only if'' direction. A second corollary records the corresponding statement for irreducible QPPs.