This Volume Contains Papers Selected For Presentation At The 26th International Symposium On Mathematical Foundations Of Computer Science – Mfcs 2001, Held In Mari´ansk´el´azn?e, Czech Republic, August 27 – 31, 2001. Mfcs 2001 Was Organized By The Mathematical Institute (academy Of S- Ences Of The Czech Republic), The Institute For Theoretical Computer Science (charles University, Faculty Of Mathematics And Physics), The Institute Of C- Puter Science (academy Of Sciences Of The Czech Republic), And Action M Agency. It Was Supported By The European Research Consortium For Informatics And Mathematics, The Czech Research Consortium For Informatics And Ma- Ematics, And The European Association For Theoretical Computer Science. We Gratefully Acknowledge The Support Of All These Institutions. The Series Of Mfcs Symposia, Organized On A Rotating Basis In Poland, S- Vakia, And The Czech Republic, Has A Well-established Tradition. The Aim Is To Encourage High-quality Research In All Branches Of Theoretical Computer Science And Bring Together Specialists Who Do Not Usually Meet At Specialized Confer- ? Ences. Previous Meetings Tookplace In Jablonna, 1972; Strbsk´e Pleso, 1973; J- Wisin, 1974; Marian ´ Sk´el´azn?e, 1975; Gdan ´sk, 1976; Tatransk´a Lomnica, 1977; Za- ? Kopane, 1978; Olomouc, 1979; Rydzina, 1980; Strbsk´e Pleso, 1981; Prague, 1984; Bratislava, 1986; Karlovy Vary, 1988; Porabk ¸ A-kozubnik, 1989; Bansk´abystrica, 1990; Kazimierz Dolny, 1991; Prague, 1992; Gdan ´sk, 1993; Ko?sice, 1994; Prague, 1995; Krak´ow, 1996; Bratislava, 1997; Brno, 1998; Szklarska Por¸eba, 1999; And Bratislava, 2000. Invited Talks -- A New Category For Semantics -- On Implications Between P-np-hypotheses: Decision Versus Computation In Algebraic Complexity -- Playing Games With Algorithms: Algorithmic Combinatorial Game Theory -- Some Recent Results On Data Mining And Search -- Hypertree Decompositions: A Survey -- The Strength Of Non-size-increasing Computation (introduction And Summary) -- To Recent Quantum Algorithms -- Decomposition Methods And Sampling Circuits In The Cartesian Lattice -- New Algorithms For K-sat Based On The Local Search Principle -- Linear Temporal Logic And Finite Semigroups -- Contributed Talks -- Refined Search Tree Technique For Dominating Set On Planar Graphs -- The Computational Power Of A Family Of Decision Forests -- Exact Results For Accepting Probabilities Of Quantum Automata -- Improved Bounds On The Weak Pigeonhole Principle And Infinitely Many Primes From Weaker Axioms -- Analysis Problems For Sequential Dynamical Systems And Communicating State Machines --^ The Complexity Of Tensor Circuit Evaluation -- Computing Reciprocals Of Bivariate Power Series -- Automatic Verification Of Recursive Procedures With One Integer Parameter -- Graph-driven Free Parity Bdds: Algorithms And Lower Bounds -- Computable Versions Of Baire’s Category Theorem -- Automata On Linear Orderings -- Algorithmic Information Theory And Cellular Automata Dynamics -- The K-median Problem For Directed Trees -- On Pseudorandom Generators In Nc0 -- There Are No Sparse Npw-hard Sets -- Sharing One Secret Vs. Sharing Many Secrets: Tight Bounds For The Max Improvement Ratio -- (h,c,k) -coloring: Fast, Easy, And Hard Cases -- Randomness And Reductibility -- On The Computational Complexity Of Infinite Words -- Lower Bounds For On-line Single-machine Scheduling -- Approximation Algorithms And Complexity Results For Path Problems In Trees Of Rings -- A 3-approximation Algorithm For Movement Minimization In Conveyor Flow Shop Processing --^ Quantifier Rank For Parity Of Embedded Finite Models -- Space Hierarchy Theorem Revised -- Converting Two—way Nondeterministic Unary Automata Into Simpler Automata -- The Complexity Of The Minimal Polynomial -- Note On Minimal Finite Automata -- Synchronizing Finite Automata On Eulerian Digraphs -- A Time Hierarchy For Bounded One-way Cellular Automata -- Checking Amalgamability Conditions Forcasl Architectural Specifications -- On-line Scheduling With Tight Deadlines -- Complexity Note On Mixed Hypergraphs -- News From The Online Traveling Repairman -- Word Problems For 2-homogeneous Monoids And Symmetric Logspace -- Variations On A Theorem Of Fine & Wilf -- Upper Bounds On The Bisection Width Of 3- And 4-regular Graphs -- Satisfiability Of Systems Of Equations Over Finite Monoids -- Rational Graphs Trace Context-sensitive Languages -- Towards Regular Languages Over Infinite Alphabets -- Partial Information And Special Case Algorithms --^ The Complexity Of Computing The Number Of Self-avoiding Walks In Two-dimensional Grid Graphs And In Hypercube Graphs -- From Bidirectionality To Alternation -- Syntactic Semiring Of A Language -- On Reducibility And Symmetry Of Disjoint Np-pairs -- Hierarchy Of Monotonically Computable Real Numbers -- On The Equational Definition Of The Least Prefixed Point -- On The Periods Of Partial Words -- The Size Of Power Automata -- On The Approximability Of The Steiner Tree Problem -- Alignment Between Two Rna Structures -- Characterization Of Context-free Languages With Polynomially Bounded Ambiguity. Jiří Sgall, Aleš Pultr, Petr Kolman (eds.). Includes Bibliographical References And Index.