2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/103750Let K be a function field in one variable over an arbitrary field F. Given a rational function f(z) in K(z) of degree at least two, the associated canonical height on the projective line was defined by Call and Silverman. The preperiodic points of f all have canonical height zero; conversely, if F is a finite field, then every point of canonical height zero is preperiodic. However, if F is an infinite field, then there may be non-preperiodic points of canonical height zero. In this paper, we show that for polynomial f, such points exist only if f is isotrivial. In fact, such K-rational points exist only if f is defined over the constant field of K after a K-rational change of coordinates.9 pages; added references, corrected minor typos, updated definition of isotrivial for dynamical systems, added Proposition 5.1 to clarify the main proofNumber TheoryDynamical Systems11G50 (Primary); 11D45, 37F10 (Secondary)text