Limit-point conjecture for the generalized Rogers-Ramanujan continued fraction

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Let a=ζka=\zeta_k and x=ζmx=\zeta_m be primitive roots of unity, with m≥3m\geq3. Limit-point conjecture. The continued fraction Ka(x)K_a(x) converges exactly when

14+am∉Q(a,x)and14+am∉R≤0\sqrt{\frac14+a^m}\notin\mathbb Q(a,x)\quad\text{and}\quad \frac14+a^m\notin\mathbb R_{\leq0}

and hence exactly when 5∤m5\nmid m or k∤mk\nmid m, and am≠−1a^m\neq-1. In the convergent case its limit is the one given by the paper's general limit formula. If 5∣m5\mid m and k∣mk\mid m, the two limit points are

Pm−2(a,x)12±14+am−axm−1Pm−3(a,x).\frac{P_{m-2}(a,x)}{\frac12\pm\sqrt{\frac14+a^m}-ax^{m-1}P_{m-3}(a,x)}.

For 3∣m3\mid m and am=−1a^m=-1, these limit points may also occur; otherwise, the remaining subsequences produce exactly the three additional limit points specified by the source. This conjectural picture organizes convergence, divergence, and all predicted limit points of the continued fraction at primitive roots of unity.

References

Primary source

Emil-Alexandru Ciolan and Robert Axel Neiss, “Convergence properties of the classical and generalized Rogers-Ramanujan continued fraction”, arXiv:1504.06482 (2015).

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