Bounds on the individual Betti numbers of complex varieties, stability and algorithms

Abstract: We prove graded bounds on the individual Betti numbers of affine and projective complex varieties. In particular, we give for each $p,d,r$, explicit bounds on the $p$-th Betti numbers of affine and projective subvarieties of $\mathrm{C}^k$, $\mathbb{P}^k_{\mathrm{C}}$, as well as products of projective spaces, defined by $r$ polynomials of degrees at most $d$ as a function of $p,d$ and $r$. Unlike previous bounds these bounds are independent of $k$, the dimension of the ambient space. We also prove as consequences of our technique certain homological and representational stability results for sequences of complex projective varieties which could be of independent interest. Finally, we highlight differences in computational complexities of the problem of computing Betti numbers of complex as opposed to real projective varieties.