Browsing by Author "Stober, Florian"

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    Average case considerations for Mergelnsertion
    (2018) Stober, Florian
    The MergeInsertion Algorithm, also known as Ford-Johnson Algorithm, is a sorting algorithm that was discovered by Ford and Johnson in 1959. It was later described by Knuth as MergeInsertion. The algorithm can be divided into three steps: First pairs of elements are compared. Then the larger half is sorted using MergeInsertion. And last the remaining elements are inserted. The most interesting property of this algorithm is the number of comparisons it requires, which is close to the information-theoretic lower bound. While the worst-case behavior is well understood, only little is known about the average-case. This thesis takes a closer look at the average case behavior. An upper bound of n log n − 1.4005n + o(n) is established. For small n the exact values are calculated. Furthermore the impact of different approaches to binary insertion on the number of comparisons is explored. To conclude we perform some experiments to evaluate different approaches on improving MergeInsertion.
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    On the average case of MergeInsertion
    (2020) Stober, Florian; Weiß, Armin
    MergeInsertion, also known as the Ford-Johnson algorithm, is a sorting algorithm which, up to today, for many input sizes achieves the best known upper bound on the number of comparisons. Indeed, it gets extremely close to the information-theoretic lower bound. While the worst-case behavior is well understood, only little is known about the average case. This work takes a closer look at the average case behavior. In particular, we establish an upper bound of nlogn-1.4005n+o(n) comparisons. We also give an exact description of the probability distribution of the length of the chain a given element is inserted into and use it to approximate the average number of comparisons numerically. Moreover, we compute the exact average number of comparisons for n up to 148. Furthermore, we experimentally explore the impact of different decision trees for binary insertion. To conclude, we conduct experiments showing that a slightly different insertion order leads to a better average case and we compare the algorithm to Manacher’s combination of merging and MergeInsertion as well as to the recent combined algorithm with (1,2)-Insertionsort by Iwama and Teruyama.
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    The power word problem in graph groups
    (2021) Stober, Florian
    This thesis studies the complexity of the power word problem in graph groups. The power word problem is a variant of the word problem, where the input is a power word. A power word is a compact representation of a word. It may contain powers p^x, where p is a finite word and x is a binary encoded integer. A graph group, also known as right-angled Artin group or partially commutative group is a free group augmented with commutation relations. We show that the power word problem in graph groups can be decided in polynomial time, and more precisely it is AC^0-Turing-reducible to the word problem of the free group with two generators F_2. Being a generalization of graph groups, we also look into the power word problem in graph products. The power word problem in a fixed graph product is AC^0-Turing-reducible to the word problem of the free group F_2 and the power word problem of the base groups. Furthermore, we look into the uniform power word problem in a graph product, where the dependence graph and the base groups are part of the input. Given a class of finitely generated groups C, the uniform power word problem in a graph product is CL-Turing-reducible to the word problem in the free group F_2 and the uniform power word problem in C. Finally, we show that as a consequence of our results on the power word problem the uniform knapsack problem in graph groups is NP-complete.
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    The power word problem in graph products
    (2024) Lohrey, Markus; Stober, Florian; Weiß, Armin
    The power word problem for a group Gasks whether an expression u1x1⋯unxn, where the uiare words over a finite set of generators of Gand the xibinary encoded integers, is equal to the identity of G. It is a restriction of the compressed word problem, where the input word is represented by a straight-line program (i.e., an algebraic circuit over G). We start by showing some easy results concerning the power word problem. In particular, the power word problem for a group Gis uNC1-many-one reducible to the power word problem for a finite-index subgroup of G. For our main result, we consider graph products of groups that do not have elements of order two. We show that the power word problem in a fixed such graph product is AC0-Turing-reducible to the word problem for the free group F2and the power word problems of the base groups. Furthermore, we look into the uniform power word problem in a graph product, where the dependence graph and the base groups are part of the input. Given a class of finitely generated groups Cwithout order two elements, the uniform power word problem in a graph product can be solved in AC0[C=LUPowWP(C)], where UPowWP(C)denotes the uniform power word problem for groups from the class C. As a consequence of our results, the uniform knapsack problem in right-angled Artin groups is NP-complete. The present paper is a combination of the two conference papers (Lohrey and Weiß 2019b, Stober and Weiß 2022a). In Stober and Weiß (2022a) our results on graph products were wrongly stated without the additional assumption that the base groups do not have elements of order two. In the present work we correct this mistake. While we strongly conjecture that the result as stated in Stober and Weiß (2022a) is true, our proof relies on this additional assumption.