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- Johannes Köbler, Sebastian Kuhnert, Oleg Verbitsky
- J. Discrete Algorithms
- 2012

We present a logspace algorithm that constructs a canonical intersection model for a given proper circular-arc graph, where canonical means that models of isomorphic graphs are equal. This implies that the recognition and the isomorphism problems for this class of graphs are solvable in logspace. For a broader class of concave-round graphs, that still… (More)

- Birgit Jenner, Johannes Köbler, Pierre McKenzie, Jacobo Torán
- J. Comput. Syst. Sci.
- 2003

- Johannes Köbler, Uwe Schöning, Jacobo Torán
- Acta Informatica
- 1988

We introduce a new class of functions, called span functions which count the different output values that occur at the leaves of the computation tree associated with a nondeterministic polynomial time Turing machine transducer. This function class has natural complete problems; it is placed between Valiant's function classes # P and # NP, and contains both… (More)

- Johannes Köbler, Osamu Watanabe
- ICALP
- 1995

We show that if a self-reducible set has polynomial-size circuits, then it is low for the probabilistic class ZPP(NP). As a consequence we get a deeper collapse of the polynomial-time hierarchy PH to ZPP(NP) under the assumption that NP has polynomial-size circuits. This improves on the well-known result of Karp, Lipton, and Sipser [KL80] stating a collapse… (More)

- Johannes Köbler, Uwe Schöning, Klaus W. Wagner
- ITA
- 1987

- Johannes Köbler, Jochen Messner, Jacobo Torán
- Inf. Comput.
- 2003

A polynomial time computable function ¦ £ ¦ £ whose range is a set Ä is called a proof system for Ä. In this setting, an-proof for Ü ¾ Ä is just a string Û with´Ûµ Ü. Cook and Reckhow defined this concept in [13], and in order to compare the relative strength of different proof systems for the set TAUT of tautologies in propositional logic, they considered… (More)

- Johannes Köbler
- Theor. Comput. Sci.
- 1993

The low hierarchy within NP and the extended low hierarchy h a ve turned out to be very useful in classifying many i n teresting language classes. We relocate P=poly from the third-level EL P 3 (Balcc azar et al., 1986) to the third-level EL P 3 of the extended low hierarchy. The location of P=poly in EL P 3 is optimal since, as shown by Allender and… (More)

- Johannes Köbler, Seinosuke Toda
- Mathematical systems theory
- 1993

We investigate the computational power of the new counting class ModP which generalizes the classes Mod p P,p prime. We show that ModP is polynomialtime truth-table equivalent in power to #P and that ModP is contained in the class AmpMP. As a consequence, the classes PP, ModP, and AmpMP are all Turing equivalent, and thus AmpMP and ModP are not low for MP… (More)

- Johannes Köbler, Sebastian Kuhnert
- MFCS
- 2009

We show that, for k constant, k-tree isomorphism can be decided in logarithmic space by giving a logspace canonical labeling algorithm. The algorithm computes a unique tree decomposition, uses colors to fully encode the structure of the original graph in the decomposition tree and invokes Lindell's tree canonization algorithm. As a consequence, the… (More)

- Frederic Green, Johannes Köbler, Kenneth W. Regan, Thomas Schwentick, Jacobo Torán
- J. Comput. Syst. Sci.
- 1995

We introduce the class MP of languages L which can be solved in polynomial time with the additional information of one bit from a #P function f. We prove that the polynomial hierarchy and the classes Mod k P, k ≥ 2, are low for this class. We show that the middle bit of f (x) is as powerful as any other bit, and that a wide range of bits around the middle… (More)