12,789 research outputs found
Combinatorial Solutions to Normal Ordering of Bosons
We present a combinatorial method of constructing solutions to the normal
ordering of boson operators. Generalizations of standard combinatorial notions
- the Stirling and Bell numbers, Bell polynomials and Dobinski relations - lead
to calculational tools which allow to find explicitly normally ordered forms
for a large class of operator functions.Comment: Presented at 14th Int. Colloquium on Integrable Systems, Prague,
Czech Republic, 16-18 June 2005. 6 pages, 11 reference
From Quantum Mechanics to Quantum Field Theory: The Hopf route
We show that the combinatorial numbers known as {\em Bell numbers} are
generic in quantum physics. This is because they arise in the procedure known
as {\em Normal ordering} of bosons, a procedure which is involved in the
evaluation of quantum functions such as the canonical partition function of
quantum statistical physics, {\it inter alia}. In fact, we shall show that an
evaluation of the non-interacting partition function for a single boson system
is identical to integrating the {\em exponential generating function} of the
Bell numbers, which is a device for encapsulating a combinatorial sequence in a
single function. We then introduce a remarkable equality, the Dobinski
relation, and use it to indicate why renormalisation is necessary in even the
simplest of perturbation expansions for a partition function. Finally we
introduce a global algebraic description of this simple model, giving a Hopf
algebra, which provides a starting point for extensions to more complex
physical systems
A generic Hopf algebra for quantum statistical mechanics
In this paper, we present a Hopf algebra description of a bosonic quantum
model, using the elementary combinatorial elements of Bell and Stirling
numbers. Our objective in doing this is as follows. Recent studies have
revealed that perturbative quantum field theory (pQFT) displays an astonishing
interplay between analysis (Riemann zeta functions), topology (Knot theory),
combinatorial graph theory (Feynman diagrams) and algebra (Hopf structure).
Since pQFT is an inherently complicated study, so far not exactly solvable and
replete with divergences, the essential simplicity of the relationships between
these areas can be somewhat obscured. The intention here is to display some of
the above-mentioned structures in the context of a simple bosonic quantum
theory, i.e. a quantum theory of non-commuting operators that do not depend on
space-time. The combinatorial properties of these boson creation and
annihilation operators, which is our chosen example, may be described by
graphs, analogous to the Feynman diagrams of pQFT, which we show possess a Hopf
algebra structure. Our approach is based on the quantum canonical partition
function for a boson gas.Comment: 8 pages/(4 pages published version), 1 Figure. arXiv admin note: text
overlap with arXiv:1011.052
Euclidean asymptotic expansions of Green functions of quantum fields (II) Combinatorics of the asymptotic operation
The results of part I (hep-ph/9612284) are used to obtain full asymptotic
expansions of Feynman diagrams renormalized within the MS-scheme in the regimes
when some of the masses and external momenta are large with respect to the
others. The large momenta are Euclidean, and the expanded diagrams are regarded
as distributions with respect to them. The small masses may be equal to zero.
The asymptotic operation for integrals is defined and a simple combinatorial
techniques is developed to study its exponentiation. The asymptotic operation
is used to obtain the corresponding expansions of arbitrary Green functions.
Such expansions generalize and improve upon the well-known short-distance
operator-product expansions, the decoupling theorem etc.; e.g. the low-energy
effective Lagrangians are obtained to all orders of the inverse heavy mass. The
obtained expansions possess the property of perfect factorization of large and
small parameters, which is essential for meaningful applications to
phenomenology. As an auxiliary tool, the inversion of the R-operation is
constructed. The results are valid for arbitrary QFT models.Comment: one .sty + one .tex (LaTeX 2.09) + one .ps (GSview) = 46 pp. Many
fewer misprints than the journal versio
Exponential Operators, Dobinski Relations and Summability
We investigate properties of exponential operators preserving the particle
number using combinatorial methods developed in order to solve the boson normal
ordering problem. In particular, we apply generalized Dobinski relations and
methods of multivariate Bell polynomials which enable us to understand the
meaning of perturbation-like expansions of exponential operators. Such
expansions, obtained as formal power series, are everywhere divergent but the
Pade summation method is shown to give results which very well agree with exact
solutions got for simplified quantum models of the one mode bosonic systems.Comment: Presented at XIIth Central European Workshop on Quantum Optics,
Bilkent University, Ankara, Turkey, 6-10 June 2005. 4 figures, 6 pages, 10
reference
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