#### Filter Results:

- Full text PDF available (66)

#### Publication Year

1964

2017

#### Publication Type

#### Co-author

#### Publication Venue

#### Key Phrases

Learn More

- 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)

- Johannes Köbler, Uwe Schöning, Jacobo Torán
- CAAP
- 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

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

- 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, Uwe Schöning, Jacobo Torán
- Computational Complexity
- 1992

We show that the graph isomorphism problem is low for PP and for C = P, i.e., it does not provide a PP or C = P computation with any additional power when used as an oracle. Furthermore, we show that graph isomorphism belongs to the class LWPP (see Fenner, Fortnow, Kurtz [12]). A similar result holds for the (apparently more difficult) problem Group… (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
- Structure in Complexity Theory Conference
- 1993

We i n vestigate the computational power of the new counting class ModP which generalizes the classes Mod p P, p prime. We show that ModP is polynomial-time truth-table equivalent i n p o wer to #P and that ModP i s contained in the class AmpMP. As a consequence, the classes PP, ModP a n d AmpMP are all Turing equivalent, and thus AmpMP and ModP are not low… (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)