Exclusion of the (8,28) Newton-Polygon Case in the Plane Jacobian Conjecture

Guccione, Guccione, Horruitiner, and Valqui proved that a hypothetical plane Jacobian counterexample of maximum degree below 125 must have degree pair (72,108), up to interchange, and reduced that pair to two Newton polygon alternatives called the (8, 28) case. Using that reduction, we give an exact computer-assisted exclusion of both alternatives. Combined with their theorem, this removes the last candidate at the previously attained bound 108 and raises the maximum-degree lower bound for a possible counterexample from 108 to 125. The common top-edge equation has exactly five dessin classes. For the smaller polygon, exact elimination over a degree-five number field produces a Bézout identity. For the larger polygon, descending coefficient elimination and a weighted Macaulay matrix produce a labelled 261 × 261 minor with determinant 21 modulo 23. Small standard-library checkers verify the final proof objects and negative controls. The formal inputs, generators, proof objects, and canonical-byte audits are distributed with the manuscript.