7,388 research outputs found
In the Maze of Data Languages
In data languages the positions of strings and trees carry a label from a
finite alphabet and a data value from an infinite alphabet. Extensions of
automata and logics over finite alphabets have been defined to recognize data
languages, both in the string and tree cases. In this paper we describe and
compare the complexity and expressiveness of such models to understand which
ones are better candidates as regular models
The nitrite anion: the key intermediate in alkyl nitrates degradative mechanism.
Alkyl nitrates, _in vivo_, are metabolized to yield nitric oxide, and thiol groups are considered necessary cofactors. This statement is based on studies that underline how these species potentiate hemodynamic responsiveness to nitrates in patients with ischemic heart disease. However, the role of thiols might be mediated by the formation of corresponding S-nitrosothiols, and a redox process is responsible for the nitrates' degradation: an enzyme, probably the cytochrome P450, is involved _in vivo_. Here, we report evidence that, in vitro, no reaction between thiols and alkyl nitrates takes place, but that stronger reducing agents, such as iron (II) derivatives, are necessary: alkoxy radicals and the nitrite anion are the reaction intermediates. The latter, in slightly acidic conditions, for instance mimicking ischemic conditions, is shown to nitrosilate thiols to the corresponding S-nitrosothiols: the real NO suppliers. Therefore, the direct release of NO from nitrates is excluded. Finally, the in vivo role of thiols on depletion and tolerance is also accounted for
Quantum Parametric Resonance of a dissipative oscillator: fading and persistent memory in the long-time evolution
The evolution of a quantum oscillator, with periodically varying frequency
and damping, is studied in the two cases of parametric resonance (PR) producing
a limited, or unlimited stretching of the wave function. The different
asymptotic behaviors of the energy distribution, show the non trivial interplay
between PR and the initial quantum state. In the first case, the oscillator's
mean energy tends asymptotically to a fully classical value, independent of the
initial state, with vanishing relative quantum fluctuations. In the second
case, the memory of the initial state persists over arbitrary long time scales,
both in the mean value and in the large quantum fluctuations of the energy.Comment: 20 pages, 2 figure
Bogoliubov theory of interacting bosons: new insights from an old problem
In a gas of interacting bosons, the Hamiltonian , obtained by
dropping all the interaction terms between free bosons with moment
, is diagonalized exactly. The resulting
eigenstates depend on two discrete indices
, where numerates the \emph{quasiphonons}
carrying a moment , responsible for transport or dissipation
processes. , in turn, numerates a ladder of \textquoteleft
vacua\textquoteright, with increasing
equispaced energies, formed by boson pairs with opposite moment. Passing from
one vacuum to another (), results from
creation/annihilation of new momentless collective excitations, that we call
\emph{vacuons}. Exact quasiphonons originate from one of the vacua by
\textquoteleft creating\textquoterightan asymmetry in the number of
opposite moment bosons. The well known Bogoliubov collective excitations (CEs)
are shown to coincide with the exact eigenstates
, i.e. with the quasiphonons created from
the lowest-level vacuum (). All this is discussed, in view of existing or
future experimental observations of the vacuons (PBs), a sort of bosonic Cooper
pairs, which are the main factor of novelty beyond Bogoliubov theory.Comment: 13 pages, 1 figur
Exact canonic eigenstates of the truncated Bogoliubov Hamiltonian in an interacting bosons gas
In a gas of weakly interacting bosons \cite{Bogo1, Bogo2}, a truncated
canonic Hamiltonian follows from dropping all the interaction
terms between free bosons with momentum .
Bogoliubov Canonic Approximation (BCA) is a further manipulation, replacing the
number \emph{operator} of free particles in
, with the total number of bosons. BCA transforms
into a different Hamiltonian
,
where and create/annihilate non
interacting pseudoparticles. The problem of the \emph{exact} eigenstates of the
truncated Hamiltonian is completely solved in the thermodynamic limit (TL) for
a special class of eigensolutions , denoted as
\textquoteleft s-pseudobosons\textquoteright, with energies
and \emph{zero} total momentum. Some preliminary results
are given for the exact eigenstates (denoted as \textquoteleft
-pseudobosons\textquoteright), carrying a total momentum
(). A comparison is done with
and with the Gross-Pitaevskii theory (GPT), showing that some
differences between exact and BCA/GPT results persist even in the TL. Finally,
it is argued that the emission of -pseudobosons, which is responsible for
the dissipation \emph{la} Landau \cite{L}, could be significantly
different from the usual picture, based on BCA pseudobosons
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