4,733 research outputs found
Restricted -Stirling Numbers and their Combinatorial Applications
We study set partitions with distinguished elements and block sizes found
in an arbitrary index set . The enumeration of these -partitions
leads to the introduction of -Stirling numbers, an extremely
wide-ranging generalization of the classical Stirling numbers and the
-Stirling numbers. We also introduce the associated -Bell and
-factorial numbers. We study fundamental aspects of these numbers,
including recurrence relations and determinantal expressions. For with some
extra structure, we show that the inverse of the -Stirling matrix
encodes the M\"obius functions of two families of posets. Through several
examples, we demonstrate that for some the matrices and their inverses
involve the enumeration sequences of several combinatorial objects. Further, we
highlight how the -Stirling numbers naturally arise in the enumeration
of cliques and acyclic orientations of special graphs, underlining their
ubiquity and importance. Finally, we introduce related generalizations
of the poly-Bernoulli and poly-Cauchy numbers, uniting many past works on
generalized combinatorial sequences
Normal Order: Combinatorial Graphs
A conventional context for supersymmetric problems arises when we consider
systems containing both boson and fermion operators. In this note we consider
the normal ordering problem for a string of such operators. In the general
case, upon which we touch briefly, this problem leads to combinatorial numbers,
the so-called Rook numbers. Since we assume that the two species, bosons and
fermions, commute, we subsequently restrict ourselves to consideration of a
single species, single-mode boson monomials. This problem leads to elegant
generalisations of well-known combinatorial numbers, specifically Bell and
Stirling numbers. We explicitly give the generating functions for some classes
of these numbers. In this note we concentrate on the combinatorial graph
approach, showing how some important classical results of graph theory lead to
transparent representations of the combinatorial numbers associated with the
boson normal ordering problem.Comment: 7 pages, 15 references, 2 figures. Presented at "Progress in
Supersymmetric Quantum Mechanics" (PSQM'03), Valladolid, Spain, July 200
Combinatorial Physics, Normal Order and Model Feynman Graphs
The general normal ordering problem for boson strings is a combinatorial
problem. In this note we restrict ourselves to single-mode boson monomials.
This problem leads to elegant generalisations of well-known combinatorial
numbers, such as Bell and Stirling numbers. We explicitly give the generating
functions for some classes of these numbers. Finally we show that a graphical
representation of these combinatorial numbers leads to sets of model field
theories, for which the graphs may be interpreted as Feynman diagrams
corresponding to the bosons of the theory. The generating functions are the
generators of the classes of Feynman diagrams.Comment: 9 pages, 4 figures. 12 references. Presented at the Symposium
'Symmetries in Science XIII', Bregenz, Austria, 200
Combinatorial approach to generalized Bell and Stirling numbers and boson normal ordering problem
We consider the numbers arising in the problem of normal ordering of
expressions in canonical boson creation and annihilation operators. We treat a
general form of a boson string which is shown to be associated with
generalizations of Stirling and Bell numbers. The recurrence relations and
closed-form expressions (Dobiski-type formulas) are obtained for these
quantities by both algebraic and combinatorial methods. By extensive use of
methods of combinatorial analysis we prove the equivalence of the
aforementioned problem to the enumeration of special families of graphs. This
link provides a combinatorial interpretation of the numbers arising in this
normal ordering problem.Comment: 10 pages, 5 figure
Laguerre-type derivatives: Dobinski relations and combinatorial identities
We consider properties of the operators D(r,M)=a^r(a^\dag a)^M (which we call
generalized Laguerre-type derivatives), with r=1,2,..., M=0,1,..., where a and
a^\dag are boson annihilation and creation operators respectively, satisfying
[a,a^\dag]=1. We obtain explicit formulas for the normally ordered form of
arbitrary Taylor-expandable functions of D(r,M) with the help of an operator
relation which generalizes the Dobinski formula. Coherent state expectation
values of certain operator functions of D(r,M) turn out to be generating
functions of combinatorial numbers. In many cases the corresponding
combinatorial structures can be explicitly identified.Comment: 14 pages, 1 figur
Combinatorially interpreting generalized Stirling numbers
Let be a word in alphabet with 's and 's.
Interpreting "" as multiplication by , and "" as differentiation with
respect to , the identity , valid
for any smooth function , defines a sequence , the terms of
which we refer to as the {\em Stirling numbers (of the second kind)} of .
The nomenclature comes from the fact that when , we have , the ordinary Stirling number of the second kind.
Explicit expressions for, and identities satisfied by, the have been
obtained by numerous authors, and combinatorial interpretations have been
presented. Here we provide a new combinatorial interpretation that retains the
spirit of the familiar interpretation of as a count of
partitions. Specifically, we associate to each a quasi-threshold graph
, and we show that enumerates partitions of the vertex set of
into classes that do not span an edge of . We also discuss some
relatives of, and consequences of, our interpretation, including -analogs
and bijections between families of labelled forests and sets of restricted
partitions.Comment: To appear in Eur. J. Combin., doi:10.1016/j.ejc.2014.07.00
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
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