## On the Betti numbers of sign conditions

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- by Saugata Basu, Richard Pollack and Marie-Françoise Roy
- Proc. Amer. Math. Soc.
**133**(2005), 965-974 - DOI: https://doi.org/10.1090/S0002-9939-04-07629-4
- Published electronically: November 19, 2004
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## Abstract:

Let $\mathrm {R}$ be a real closed field and let ${\mathcal Q}$ and ${\mathcal P}$ be finite subsets of $\mathrm {R}[X_1,\ldots ,X_k]$ such that the set ${\mathcal P}$ has $s$ elements, the algebraic set $Z$ defined by $\bigwedge _{Q \in {\mathcal Q}}Q=0$ has dimension $k’$ and the elements of${\mathcal Q}$ and ${\mathcal P}$ have degree at most $d$. For each $0 \leq i \leq k’,$ we denote the sum of the $i$-th Betti numbers over the realizations of all sign conditions of ${\mathcal P}$ on $Z$ by $b_i({\mathcal P},{\mathcal Q})$. We prove that \[ b_i({\mathcal P},{\mathcal Q}) \le \sum _{j=0}^{k’ - i} {s \choose j} 4^{j} d(2d-1)^{k-1}. \] This generalizes to all the higher Betti numbers the bound ${s \choose k’}O(d)^k$ on $b_0({\mathcal P},{\mathcal Q})$. We also prove, using similar methods, that the sum of the Betti numbers of the intersection of $Z$ with a closed semi-algebraic set, defined by a quantifier-free Boolean formula without negations with atoms of the form $P \geq 0$ or $P\leq 0$ for $P\in {\mathcal P}$, is bounded by \[ \sum _{i = 0}^{k’}\sum _{j = 0}^{k’ - i} {s \choose j} 6^{j} d(2d-1)^{k-1}, \] making the bound $s^{k’} O(d)^k$ more precise.## References

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## Bibliographic Information

**Saugata Basu**- Affiliation: School of Mathematics, Georgia Institute of Technology, Atlanta, Georgia 30332
- MR Author ID: 351826
- Email: saugata@math.gatech.edu
**Richard Pollack**- Affiliation: Courant Institute of Mathematical Sciences, New York University, New York, New York 10012
- Email: pollack@cims.nyu.edu
**Marie-Françoise Roy**- Affiliation: IRMAR (URA CNRS 305), Université de Rennes, Campus de Beaulieu 35042 Rennes cedex, France
- Email: mfroy@maths.univ-rennes1.fr
- Received by editor(s): July 3, 2002
- Received by editor(s) in revised form: October 10, 2003
- Published electronically: November 19, 2004
- Additional Notes: The first author was supported in part by NSF grant CCR-0049070 and an NSF Career Award 0133597.

The second author was supported in part by NSA grant MDA904-01-1-0057 and NSF grants CCR-9732101 and CCR-0098246. - Communicated by: Michael Stillman
- © Copyright 2004 American Mathematical Society
- Journal: Proc. Amer. Math. Soc.
**133**(2005), 965-974 - MSC (2000): Primary 14P10; Secondary 14P25
- DOI: https://doi.org/10.1090/S0002-9939-04-07629-4
- MathSciNet review: 2117195