On the Decomposition of Generalized Fermat Varieties in \(P^{3}\) Corresponding to Kasami-Welch Functions

Moisés Delgado, H Janwa, Moises Delgado1, Heeralal Janwa2
1Department of Mathematics University of Puerto Rico — Cayey Campus Cayey, Puerto Rico, 00727 USA
2Department of Mathematics, Faculty of Natural Sciences University of Puerto Rico ~ Rio Piedras Campus San Juan, Puerto Rico, 00931 USA

Abstract

The study of the generalized Fermat variety

\[
\phi_j = \frac{x^j + y^j + z^j + (x+y+z)^j}{(x+y)(x+z)(y+z)}
\]

defined over a finite field \( L = \mathbb{F}_q \), where \( q = 2^n \) for some positive integer \( n \), plays an important role in the study of (APN) functions and exceptional APN functions. This study arose after a characterization by Rodier that relates these functions with the number of rational points of \( \phi_j = (x,y,z) \). The most studied cases are when \( j = 2^k + 1 \) and \( j = 2^{2k} – 2^{k} + 1 \), the Gold and Kasami-Welch numbers. In this article, we make a claim about the decomposition of \( \phi_j \) into absolutely irreducible components. If these components intersect transversally at a particular point, then the corresponding Kasami-Welch polynomial is absolutely irreducible. This implies that the function is not exceptional APN, thus helping us make progress on the stated conjecture.

Keywords: APN functions, exceptional APN functions, absolutely irreducible polynomials, transversal intersection, exceptional APN con- jecture.