SCHUR COVERS and CARLITZ’S CONJECTURE

We use the classification of finite simple groups and covering theory in positive characteristic to solve Carlitz’s conjecture (1966). An exceptional polynomial f over a finite field Fq is a polynomial that is a permutation polynomial on infinitely many finite extensions of Fq. Carlitz’s conjecture says f must be of odd degree (if q is odd). Indeed, excluding characteristic 2 and 3, arithmetic monodromy groups of exceptional polynomials must be affine groups. We don’t, however, know which affine groups appear as the geometric monodromy group of exceptional polynomials. Thus, there remain unsolved problems. Riemann’s existence theorem in positive characteristic will surely play a role in their solution. We have, however, completely classified the exceptional polynomials of degree equal to the characteristic. This solves a problem from Dickson’s thesis (1896). Further, we generalize Dickson’s problem to include a description of all known exceptional polynomials. Finally: The methods allow us to consider covers X → P that generalize the notion of exceptional polynomials. These covers have this property: Over each Fqt point of P there is exactly one Fqt point of X for infinitely many t. Thus X has a rare diophantine property when X has genus greater than 0. It has exactly q +1 points in Fqt for infinitely many t. This gives exceptional covers a special place in the theory of counting rational points on curves over finite fields explicitly. Corollary 14.2 holds also for an indecomposable exceptional cover having (at least) one totally ramified place over a rational point of the base. Its arithmetic monodromy group is an affine group. • Supported by NSA grant MDA 14776 and BSF grant 87-00038. •2 First author supported by the Institute for Advanced Studies in Jerusalem and IFR Grant #90/91-15. •3 Supported by NSF grant DMS 91011407. Dedication: To the contributions of John Thompson to the classification of finite simple groups; and to the memory of Daniel Gorenstein and the success of his project to complete the classification. AMS Subject classification: 11G20, 12E20, 12F05, 12F10, 20B20, 20D05