Stability Regions and Bifurcations for Higher-Order Fractional Difference Equations
We study stability regions for the higher-order, two-term fractional difference equation $Δ^αx(t) + a\,Δ^βx(t + α- β- 1) = (b - 1)x(t + α- 2)$, where $0 < β\leq 1 < α\leq 2$, $a > 0$, and $b \in \mathbb{C}$. The Z-transform yields a characteristic function whose image of the unit circle determines the stability boundary. Using a winding-number formulation, we give a necessary and sufficient root-count condition for asymptotic stability. Two analytically derived parameter values, $a_1 = 2^{α-β}$ and $a_2 = 2^{α-β}(4 - α)/(2 - β)$, characterize an endpoint collision and a loss of regularity of the boundary curve, respectively. The real-parameter case and a nonlinear higher-order logistic map are treated as consequences of the same stability criterion. We also analyze the one-term family $Δ^αx(t) = (c - 1)x(t + α- N)$ for $N - 1 < α\leq N$. A winding-number bound proves that its stability region is empty for every $N \geq 3$. Numerical experiments illustrate the theoretical results.