We study systems of equations of the form $X_1 = f_1(X_1, \ldots, X_n), \ldots, X_n = f_n(X_1, \ldots, X_n)$ where each $f_i$ is a polynomial with nonnegative coefficients that add up to~$1$. The least nonnegative solution, say~$\mu$, of such equation systems is central to problems from various areas, like physics, biology, computational linguistics and probabilistic program verification. We give a simple and strongly polynomial algorithm to decide whether $\mu=(1,\ldots,1)$ holds. Furthermore, we present an algorithm that computes reliable sequences of lower and upper bounds on~$\mu$, converging linearly to~$\mu$. Our algorithm has these features despite using inexact arithmetic for efficiency. We report on experiments that show the performance of our algorithms.
@InProceedings{esparza_et_al:LIPIcs.STACS.2010.2468, author = {Esparza, Javier and Gaiser, Andreas and Kiefer, Stefan}, title = {{Computing Least Fixed Points of Probabilistic Systems of Polynomials}}, booktitle = {27th International Symposium on Theoretical Aspects of Computer Science}, pages = {359--370}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-16-3}, ISSN = {1868-8969}, year = {2010}, volume = {5}, editor = {Marion, Jean-Yves and Schwentick, Thomas}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2010.2468}, URN = {urn:nbn:de:0030-drops-24685}, doi = {10.4230/LIPIcs.STACS.2010.2468}, annote = {Keywords: Computing fixed points, numerical approximation, stochastic models, branching processes} }
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