On scattered linear sets of pseudoregulus type in $\mathrm{PG}(1,q^t)$

Abstract

Scattered linear sets of pseudoregulus type in $\mathrm{PG}(1,q^t)$ have been defined and investigated in [G. Lunardon, G. Marino, O. Polverino, R. Trombetti: Maximum scattered linear sets of pseudoregulus type and the Segre Variety ${\cal S}_{n,n}$. J. Algebr. Comb. 39 (2014), 807--831.; G. Donati, N. Durante: Scattered linear sets generated by collineations between pencils of lines. J. Algebr. Comb. 40 (2014), 1121-1134]. The aim of this paper is to continue such an investigation. Properties of a scattered linear set of pseudoregulus type, say $L$, are proved by means of three different ways to obtain $L$: (i) as projection of a $q$-order canonical subgeometry [G. Lunardon, O. Polverino: Translation ovoids of orthogonal polar spaces. Forum Math. 16 (2004), 663-669], (ii) as a set whose image under the field reduction map is the hypersurface of degree $t$ in $\mathrm{PG}(2t-1,q)$ studied in [M. Lavrauw, J. Sheekey, C. Zanella: On embeddings of minimum dimension of $\mathrm{PG}(n,q)\times \mathrm{PG}(n,q)$. Des. Codes Cryptogr. 74 (2015), 427-440], (iii) as exterior splash, by the correspondence described in [M. Lavrauw, J. Sheekey, C. Zanella: On embeddings of minimum dimension of $\mathrm{PG}(n,q)\times \mathrm{PG}(n,q)$. Des. Codes Cryptogr. 74 (2015), 427-440]. In particular, given a canonical subgeometry $\Sigma$ of $\mathrm{PG}(t-1,q^t)$, necessary and sufficient conditions are given for the projection of $\Sigma$ with center a $(t-3)$-subspace to be a linear set of pseudoregulus type. Furthermore, the $q$-order sublines are counted and geometrically described.