English

Fixed-point lifting and ghost periodic points for Chebyshev polynomials modulo odd prime powers

Number Theory 2026-05-07 v1 Dynamical Systems

Abstract

Let pp be an odd prime, let n2n\ge2, and let the nnth Chebyshev polynomial TnT_n act on Z/pkZ\Z/p^k\Z. We count fixed and exact-periodic points, allowing non-permutation degrees, and organize the finite-field formulas by the two source groups needed for prime-power lifting. Over \Fp\Fp we record the four-GCD fixed-point formula N1=gcd(n1,p1)+gcd(n+1,p1)+gcd(n1,p+1)+gcd(n+1,p+1)2δ2, N_1=\frac{\gcd(n-1,p-1)+\gcd(n+1,p-1)+\gcd(n-1,p+1)+\gcd(n+1,p+1)-2\delta}{2}, where δ=gcd(n1,2)\delta=\gcd(n-1,2). The proof separates split and nonsplit source groups for a=(ζ+ζ1)/2a=(\zeta+\zeta^{-1})/2 and counts degenerate fixed residues branch-wise. For every odd pp, N2=N1+d(p1). N_2=N_1+d(p-1). Here dd denotes the number of fixed residue classes a\Fpa\in\Fp for which Tn(a)1(modp)T_n'(a)\equiv1\pmod p. For p5p\ge5 and all k1k\ge1, Nk=N1+d(pmin(k1,\nup(n21))1). N_k=N_1+d\bigl(p^{\min(k-1,\nup(n^2-1))}-1\bigr). This all-level formula does not extend unchanged to p=3p=3, where boundary pp-adic estimates at a=±1a=\pm1 can fail; the first-lift formula remains valid. For periods, we use the Chebyshev order \corde(n)=min{r1:nr±1(mode)}. \cord_e(n)=\min\{r\ge1:n^r\equiv\pm1\pmod e\}. A source-order-ee point is periodic over \Fp\Fp exactly when gcd(n,e)=1\gcd(n,e)=1, with period \corde(n)\cord_e(n). M\"obius inversion for the iterates TnjT_{n^j} gives exact-period point counts over Z/pkZ\Z/p^k\Z for all odd pp; for p5p\ge5, the all-level fixed-point formula gives closed forms. When pnp\nmid n, orbitwise lifting modulo p2p^2 gives either full period retention or one Hensel lift plus ghost periodic points of period \cordep(n)\cord_{ep}(n). For p5p\ge5, higher lifts above a periodic residue are governed by the tower \cordepq(n)\cord_{ep^q}(n).

Cite

@article{arxiv.2605.04417,
  title  = {Fixed-point lifting and ghost periodic points for Chebyshev polynomials modulo odd prime powers},
  author = {Chatchawan Panraksa and Aram Tangboonduangjit},
  journal= {arXiv preprint arXiv:2605.04417},
  year   = {2026}
}

Comments

36 pages

R2 v1 2026-07-01T12:52:02.359Z