Let normalR be a real closed field and Q1,…,Qℓ∈R[X1,…,Xk] such that for each i,1⩽i⩽ℓ, deg(Qi)⩽di. For 1⩽i⩽ℓ, denote by MJX-tex-caligraphicscriptQi={Q1,…,Qi}, Vi the real variety defined by Qi, and ki an upper bound on the real dimension of Vi (by convention V0=Rk and k0=k). Suppose also that
2⩽d1⩽d2⩽1k+1d3⩽1(k+1)2d4⩽⋯⩽1(k+1)ℓ−3dℓ−1⩽1(k+1)ℓ−2dℓ,
and that ℓ⩽k. We prove that the number of semi‐algebraically connected components of Vℓ is bounded by
O(k)2k∏1⩽j<ℓdjkj−1−kjdℓkℓ−1.
This bound can be seen as a weak extension of the classical Bezout inequality (which holds only over algebraically closed fields and is false over real closed fields) to varieties defined over real closed fields.
Additionally, if P⊂R[X1,…,Xk] is a finite family of polynomials with deg(P)⩽d for all P∈P, cardP=s and dℓ⩽1k+1d, then we prove that the number of semi‐algebraically connected components of the realizations of all realizable sign conditions of the family MJX-tex-caligraphicscriptP restricted to Vℓ is bounded by
O(k)2k(sd)kℓ∏1⩽j⩽ℓdjkj−1−kj.
These results have found applications in discrete geometry, for proving incidence bounds [, ], as well as in efficient range searching [].