TY - JOUR
AB - We show that the fixed alphabet shortest common supersequence (SCS) and the fixed alphabet longest common subsequence (LCS) problems parameterized in the number of strings are W[1]-hard. Unless W[1]=FPT, this rules out the existence of algorithms with time complexity of O(f(k)nα) for those problems. Here n is the size of the problem instance, α is constant, k is the number of strings and f is any function of k. The fixed alphabet version of the LCS problem is of particular interest considering the importance of sequence comparison (e.g. multiple sequence alignment) in the fixed length alphabet world of DNA and protein sequences.
AU - Krzysztof Pietrzak
ID - 3209
IS - 4
JF - Journal of Computer and System Sciences
TI - On the parameterized complexity of the fixed alphabet shortest common supersequence and longest common subsequence problems
VL - 67
ER -
TY - CONF
AB - Luby and Rackoff showed how to construct a (super-)pseudo-random permutation {0,1}2n→ {0,1}2n from some number r of pseudo-random functions {0,1}n → {0,1}n. Their construction, motivated by DES, consists of a cascade of r Feistel permutations. A Feistel permutation 1for a pseudo-random function f is defined as (L, R) → (R,L ⊕ f (R)), where L and R are the left and right part of the input and ⊕ denotes bitwise XOR or, in this paper, any other group operation on {0,1}n. The only non-trivial step of the security proof consists of proving that the cascade of r Feistel permutations with independent uniform random functions {0,1}n → {0,1}n, denoted Ψ2nr is indistinguishable from a uniform random permutation {0,1}2n → {0,1}2n by any computationally unbounded adaptive distinguisher making at most O(2cn) combined chosen plaintext/ciphertext queries for any c < α, where a is a security parameter. Luby and Rackoff proved α = 1/2 for r = 4. A natural problem, proposed by Pieprzyk is to improve on α for larger r. The best known result, α = 3/4 for r = 6, is due to Patarin. In this paper we prove a = 1 -O(1/r), i.e., the trivial upper bound α = 1 can be approached. The proof uses some new techniques that can be of independent interest.
AU - Maurer, Ueli M
AU - Krzysztof Pietrzak
ID - 3210
TI - The security of many round Luby Rackoff pseudo random permutations
VL - 2656
ER -
TY - CONF
AU - Bollenbach, Tobias
AU - Strother, T.
AU - Bauer, Wolfgang
ID - 3425
TI - 3D supernova collapse calculations
VL - 166
ER -
TY - CHAP
AU - Peter Jonas
AU - Unsicker, Klaus
ED - Schmidt, R. F.
ID - 3458
T2 - Lehrbuch Vorklinik
TI - Molekulare und zelluläre Grundlagen des Nervensystems.
VL - B
ER -
TY - JOUR
AB - Neurons can produce action potentials with high temporal precision(1). A fundamental issue is whether, and how, this capability is used in information processing. According to the `cell assembly' hypothesis, transient synchrony of anatomically distributed groups of neurons underlies processing of both external sensory input and internal cognitive mechanisms(2-4). Accordingly, neuron populations should be arranged into groups whose synchrony exceeds that predicted by common modulation by sensory input. Here we find that the spike times of hippocampal pyramidal cells can be predicted more accurately by using the spike times of simultaneously recorded neurons in addition to the animals location in space. This improvement remained when the spatial prediction was refined with a spatially dependent theta phase modulation(5-8). The time window in which spike times are best predicted from simultaneous peer activity is 10-30 ms, suggesting that cell assemblies are synchronized at this timescale. Because this temporal window matches the membrane time constant of pyramidal neurons(9), the period of the hippocampal gamma oscillation(10) and the time window for synaptic plasticity(11), we propose that cooperative activity at this timescale is optimal for information transmission and storage in cortical circuits.
AU - Harris, Kenneth D
AU - Jozsef Csicsvari
AU - Hirase, Hajima
AU - Dragoi, George
AU - Buzsáki, György
ID - 3526
IS - 6948
JF - Nature
TI - Organization of cell assemblies in the hippocampus
VL - 424
ER -