Periklis A. Papakonstantinou

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Diamond-ike carbon (DLC) coatings have been investigated with respect to biocompatibility, mechanical stability under biofluid exposure, corrosion resistance and the impact of the fabrication or operation of catheter guidewires and stents upon coating integrity. High mechanical tensile and compressive forces, during guidewire winding or stent expansion,(More)
We characterize the complexity of SAT instances with path-decompositions of width w(n). Although pathwidth is the most restrictive among the studied width-parameterizations of SAT, the most time-efficient algorithms known for such SAT instances run in time 2 Ω(w(n)) , even when the path-decomposition is given in the input. We wish to better understand the(More)
We give an explicit construction of a pseudorandom generator for read-once formulas whose inputs can be read in arbitrary order. For formulas in n inputs and arbitrary gates of fan-in at most d = O(n/ log n), the pseudorandom generator uses (1 − Ω(1))n bits of randomness and produces an output that looks 2 −Ω(n)-pseudorandom to all such formulas. Our(More)
This paper consists of two conceptually related but independent parts. In the first part we initiate the study of k-SAT instances of bounded diameter. The diameter of an ordered CNF formula is defined as the maximum difference between the index of the first and the last occurrence of a variable. We investigate the relation between the diameter of a formula(More)
We ask whether an Identity Based Encryption (IBE) system can be built from simpler public-key primitives. We show that there is no black-box construction of IBE from Trapdoor Permutations (TDP) or even from Chosen Ci-phertext Secure Public Key Encryption (CCA-PKE). These black-box separation results are based on an essential property of IBE, namely that an(More)
Alekhnovich and Razborov (2002) presented an algorithm that solves SAT on instances φ of size n and tree-width TW(φ), using time and space bounded by 2 O(TW(φ)) n O(1). Although several follow-up works appeared over the last decade, the first open question of Alekhnovich and Razborov remained essentially unresolved: Can one check satisfiability of formulas(More)
The standard approach for constructing a large-stretch pseudo-random generator given a one-way permutation or given a smaller-stretch pseudo-random generator involves repeatedly composing the given primitive with itself. In this paper, we consider whether this approach is necessary , that is, whether there are constructions that do not involve composition.(More)