70,931 research outputs found
The Dimensions of Individual Strings and Sequences
A constructive version of Hausdorff dimension is developed using constructive
supergales, which are betting strategies that generalize the constructive
supermartingales used in the theory of individual random sequences. This
constructive dimension is used to assign every individual (infinite, binary)
sequence S a dimension, which is a real number dim(S) in the interval [0,1].
Sequences that are random (in the sense of Martin-Lof) have dimension 1, while
sequences that are decidable, \Sigma^0_1, or \Pi^0_1 have dimension 0. It is
shown that for every \Delta^0_2-computable real number \alpha in [0,1] there is
a \Delta^0_2 sequence S such that \dim(S) = \alpha.
A discrete version of constructive dimension is also developed using
termgales, which are supergale-like functions that bet on the terminations of
(finite, binary) strings as well as on their successive bits. This discrete
dimension is used to assign each individual string w a dimension, which is a
nonnegative real number dim(w). The dimension of a sequence is shown to be the
limit infimum of the dimensions of its prefixes.
The Kolmogorov complexity of a string is proven to be the product of its
length and its dimension. This gives a new characterization of algorithmic
information and a new proof of Mayordomo's recent theorem stating that the
dimension of a sequence is the limit infimum of the average Kolmogorov
complexity of its first n bits.
Every sequence that is random relative to any computable sequence of
coin-toss biases that converge to a real number \beta in (0,1) is shown to have
dimension \H(\beta), the binary entropy of \beta.Comment: 31 page
Unitary equivalence to a truncated Toeplitz operator: analytic symbols
Unlike Toeplitz operators on , truncated Toeplitz operators do not have
a natural matricial characterization. Consequently, these operators are
difficult to study numerically. In this note we provide criteria for a matrix
with distinct eigenvalues to be unitarily equivalent to a truncated Toeplitz
operator having an analytic symbol. This test is constructive and we illustrate
it with several examples. As a byproduct, we also prove that every complex
symmetric operator on a Hilbert space of dimension is unitarily
equivalent to a direct sum of truncated Toeplitz operators.Comment: 15 page
Effective Hausdorff Dimension in General Metric Spaces
We introduce the concept of effective dimension for a wide class of metric spaces whose metric is not necessarily based on a measure. Effective dimension was defined by Lutz (Inf. Comput., 187(1), 49–79, 2003) for Cantor space and has also been extended to Euclidean space. Lutz effectivization uses gambling, in particular the concept of gale and supergale, our extension of Hausdorff dimension to other metric spaces is also based on a supergale characterization of dimension, which in practice avoids an extra quantifier present in the classical definition of dimension that is based on Hausdorff measure and therefore allows effectivization for small time-bounds. We present here the concept of constructive dimension and its characterization in terms of Kolmogorov complexity, for which we extend the concept of Kolmogorov complexity to any metric space defining the Kolmogorov complexity of a point at a certain precision. Further research directions are indicated
Effective Continued Fraction Dimension versus Effective Hausdorff Dimension of Reals
We establish that constructive continued fraction dimension originally
defined using -gales is robust, but surprisingly, that the effective
continued fraction dimension and effective (base-) Hausdorff dimension of
the same real can be unequal in general.
We initially provide an equivalent characterization of continued fraction
dimension using Kolmogorov complexity. In the process, we construct an optimal
lower semi-computable -gale for continued fractions. We also prove new
bounds on the Lebesgue measure of continued fraction cylinders, which may be of
independent interest.
We apply these bounds to reveal an unexpected behavior of continued fraction
dimension. It is known that feasible dimension is invariant with respect to
base conversion. We also know that Martin-L\"of randomness and computable
randomness are invariant not only with respect to base conversion, but also
with respect to the continued fraction representation. In contrast, for any , we prove the existence of a real whose effective
Hausdorff dimension is less than , but whose effective continued
fraction dimension is greater than or equal to . This phenomenon is
related to the ``non-faithfulness'' of certain families of covers, investigated
by Peres and Torbin and by Albeverio, Ivanenko, Lebid and Torbin.
We also establish that for any real, the constructive Hausdorff dimension is
at most its effective continued fraction dimension
Generic pure quantum states as steady states of quasi-local dissipative dynamics
We investigate whether a generic multipartite pure state can be the unique
asymptotic steady state of locality-constrained purely dissipative Markovian
dynamics. In the simplest tripartite setting, we show that the problem is
equivalent to characterizing the solution space of a set of linear equations
and establish that the set of pure states obeying the above property has either
measure zero or measure one, solely depending on the subsystems' dimension. A
complete analytical characterization is given when the central subsystem is a
qubit. In the N-partite case, we provide conditions on the subsystems' size and
the nature of the locality constraint, under which random pure states cannot be
quasi-locally stabilized generically. Beside allowing for the possibility to
approximately stabilize entangled pure states that cannot be exact steady
states in settings where stabilizability is generic, our results offer insights
into the extent to which random pure states may arise as unique ground states
of frustration free parent Hamiltonians. We further argue that, to high
probability, pure quantum states sampled from a t-design enjoy the same
stabilizability properties of Haar-random ones as long as suitable dimension
constraints are obeyed and t is sufficiently large. Lastly, we demonstrate a
connection between the tasks of quasi-local state stabilization and unique
state reconstruction from local tomographic information, and provide a
constructive procedure for determining a generic N-partite pure state based
only on knowledge of the support of any two of the reduced density matrices of
about half the parties, improving over existing results.Comment: 36 pages (including appendix), 2 figure
Dimension Extractors and Optimal Decompression
A *dimension extractor* is an algorithm designed to increase the effective
dimension -- i.e., the amount of computational randomness -- of an infinite
binary sequence, in order to turn a "partially random" sequence into a "more
random" sequence. Extractors are exhibited for various effective dimensions,
including constructive, computable, space-bounded, time-bounded, and
finite-state dimension. Using similar techniques, the Kucera-Gacs theorem is
examined from the perspective of decompression, by showing that every infinite
sequence S is Turing reducible to a Martin-Loef random sequence R such that the
asymptotic number of bits of R needed to compute n bits of S, divided by n, is
precisely the constructive dimension of S, which is shown to be the optimal
ratio of query bits to computed bits achievable with Turing reductions. The
extractors and decompressors that are developed lead directly to new
characterizations of some effective dimensions in terms of optimal
decompression by Turing reductions.Comment: This report was combined with a different conference paper "Every
Sequence is Decompressible from a Random One" (cs.IT/0511074, at
http://dx.doi.org/10.1007/11780342_17), and both titles were changed, with
the conference paper incorporated as section 5 of this new combined paper.
The combined paper was accepted to the journal Theory of Computing Systems,
as part of a special issue of invited papers from the second conference on
Computability in Europe, 200
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