Partially complex ranks for real projective varieties

Abstract: Let $X(\mathbb {C})\subset \mathbb {P}^r(\mathbb {C})$ be an integral non-degenerate variety defined over $\mathbb {R}$. For any $q\in \mathbb {P}^r(\mathbb {R})$ we study the existence of $S\subset X(\mathbb {C})$ with small cardinality, invariant for the complex conjugation and with $q$ contained in the real linear space spanned by $S$. We discuss the advantages of these additive decompositions with respect to the $X(\mathbb {R})$-rank, i.e. the rank of $q$ with respect to $X(\mathbb {R})$. We describe the case of hypersurfaces and Veronese varieties.