ETD Collection

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Now showing 1 - 9 of 9
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    Approximation theory for exponential weights.
    (1998) Kubayi, David Giyani.
    Much of weighted polynomial approximation originated with the famous Bernstein qualitative approximation problem of 1910/11. The classical Bernstein approximation problem seeks conditions on the weight functions \V such that the set of functions {W(x)Xn};;"=l is fundamental in the class of suitably weighted continuous functions on R, vanishing at infinity. Many people worked on the problem for at least 40 years. Here we present a short survey of techniques and methods used to prove Markov and Bernstein inequalities as they underlie much of weighted polynomial approximation. Thereafter, we survey classical techniques used to prove Jackson theorems in the unweighted setting. But first we start, by reviewing some elementary facts about orthogonal polynomials and the corresponding weight function on the real line. Finally we look at one of the processes (If approximation, the Lagrange interpolation and present the most recent results concerning mean convergence of Lagrange interpolation for Freud and Erdos weights.
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    Graphs, compositions, polynomials and applications
    (2018) Ncambalala, Thokozani Paxwell
    In this thesis, we study graph compositions of graphs and two graph polynomials, the k-defect polynomials and the Hosoya polynomials. This study was motivated by the fact that it is known that the number of compositions for certain graphs can be extracted from their k-defect polynomials, for example trees and cycles. We want to investigate if these results can be extended to other classes of graphs, in particular to theta and multibridge graphs. Furthermore we want to investigate if we can mimic these results of k-defect polynomials to Hosoya polynomials of graphs. In particular, investigating if the Hosoya polynomials of graphs can be computed using, similar methods to k-defect polynomials. We start the investigation by improving the upper bound for the number of graph compositions of any graph. Thereafter, we give the exact number of graph composi- tion of theta and 4-bridge graphs. We then nd explicit expressions of the k-defect polynomials of a theta graph via its bad coloring polynomial. Furthermore, we nd explicit expressions for the Hosoya polynomials of multibridge graphs and q-vertex joins of graphs with diameter 1 and 2.
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    Anomalous dimensions for scalar operators in ABJM theory
    (2016-01-22) Kreyfelt, Rocky
    At nite N, the number of restricted Schur polynomials is greater than, or equal to the number of generalized restricted Schur polynomials. In this dissertation we study this dis- crepancy and explain its origin. We conclude that, for quiver gauge theories, in general, the generalized restricted Shur polynomials correctly account for the complete set of nite N constraints and they provide a basis, while the restricted Schur polynomials only account for a subset of the nite N constraints and are thus overcomplete. We identify several situations in which the restricted Schur polynomials do in fact account for the complete set of nite N constraints. In these situations the restricted Schur polynomials and the gen- eralized restricted Schur polynomials both provide good bases for the quiver gauge theory. Further, we demonstrate situations in which the generalized restricted Schur polynomials reduce to the restricted Schur polynomials and use these results to study the anomalous dimensions for scalar operators in ABJM theory in the SU(2) sector. The operators we consider have a classical dimension that grows as N in the large N limit. Consequently, the large N limit is not captured by summing planar diagrams { non-planar contributions have to be included. We nd that the mixing matrix at two-loop order is diagonalized using a double coset ansatz, reducing it to the Hamiltonian of a set of decoupled oscilla- tors. The spectrum of anomalous dimensions, when interpreted in the dual gravity theory, shows that the energy of the uctuations of the corresponding giant graviton is dependent on the size of the giant. The rst subleading corrections to the large N limit are also considered. These subleading corrections to the dilatation operator do not commute with the leading terms, indicating that integrability probably does not survive beyond the large N limit.
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    The computation of k-defect polynomials, suspended Y -trees and its applications
    (2015-02-06) Werner, Simon
    We start by defining a class of graphs called the suspended Y -trees and give some of its properties. We then classify all the closed sets of a general suspended Y -tree. This will lead us to counting the graph compositions of the suspended Y -tree. We then contract these closed sets one by one to obtain a set of minors for the suspended Y -trees. We will use this information to compute some of the general expression of the k-defect polynomial of a suspended Y -tree. Finally we compute the explicit Tutte polynomial of the suspended Y -trees.
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    Graphs, graph polynomials with applications to antiprisms
    (2014-07-02) Bukasa, Deborah Kembia
    The n-antiprism graph is not widely studied as a class of graphs in graph theory hence there is not much literature. We begin by de ning the n-antiprism graph and discussing properties, which we prove in the thesis, and which have not been previously presented in graph theory literature. Some of our signi cant results include proving that an n-antiprism is 4-connected, 4-edge connected and has a pathwidth of 4. A highly studied area of graph theory is the chromatic polynomial of graphs. We investigate the chromatic polynomial of the antiprism graph and attempt to nd explicit expressions for the chromatic polynomial of the antiprism graph. We express this chromatic polynomial in several forms to discover the best-suited form. We then explore the Tutte polynomial and search for an explicit expression of the Tutte polynomial of the antiprism graph. Using the relationship between a graph and its dual graph, we provide an iterative expression of the Tutte polynomial of the antiprism graph.
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    Non-planar Ads/CFT from group representation theory
    (2014-06-12) Smith, Stephanie
    In this thesis we explore certain limits of the AdS/CFT correspondence for integrability. This is done by calculating the action of the dilatation operator on operators known as restricted Schur polynomials, which are AdS/CFT dual to D3-branes known as giant gravitons. We focus on operators in N = 4 super-Yang-Mills theory, which is dual to type IIB string theory on an AdS5×S5 background. We find that, in various cases, this theory is integrable in a large N non-planar limit.
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    Brane states and group representation theory
    (2014-01-14) Nokwara, Nkululeko
    A complete understanding of quantum gravity remains an open problem. However, the AdS/CFT correspondence which relates quantum eld theories that enjoy conformal symmetry to theories of (quantum) gravity is proving to be a useful tool in shedding light on this formidable problem. Recently developed group representation theoretic methods have proved useful in understanding the large N; but non-planar limit of N = 4 supersymmetric Yang-Mills theory. In this work, we study operators that are dual to excited giant gravitons, which corresponds to a sector of N = 4 super Yang-Mills theory that is described by a large N; but non-planar limit. After a brief review of the work done in the su (2) sector, we compute the spectrum of anomalous dimensions in the su (2) sector of the Leigh-Strassler deformed theory. The result resembles the spectrum of a shifted harmonic oscillator. We then explain how to construct restricted Schur polynomials built using both fermionic and bosonic elds which transform in the adjoint of the gauge group U (N) : We show that these operators diagonalise the free eld two point function to all orders in 1=N: As an application of our new operators, we study the action of the one-loop dilatation operator in the su (2,3) sector in a large N; but non-planar limit of N = 4 super Yang-Mills theory. As in the su (2) case, the resulting spectrum matches the spectrum of a set of decoupled oscillators. Finally, in an appendix, we study the action of the one-loop dilatation operator in an sl (2) sector of N = 4 super Yang-Mills theory. Again, the resulting spectrum matches that of a set of harmonic oscillators. In all these cases, we nd that the action of the dilatation operator is diagonalised by a double coset ansatz.
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    Giant graviton oscillators
    (2013-08-07) Dessein, Matthias
    We study the action of the dilatation operator on restricted Schur polynomials labeled by Young diagrams with p long columns or p long rows. A new version of Schur-Weyl duality provides a powerful approach to the computation and manipulation of the symmetric group operators appearing in the restricted Schur polynomials. Using this new technology, we are able to evaluate the action of the one loop dilatation operator. The result has a direct and natural connection to the Gauss Law constraint for branes with a compact world volume. We find considerable evidence that the dilatation operator reduces to a decoupled set of harmonic oscillators. This strongly suggests that integrability in N = 4 super Yang-Mills theory is not just a feature of the planar limit, but extends to other large N but non-planar limits.
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    Giant graviton oscillators
    (2013-07-30) Mathwin, C. R.
    We study the action of the dilatation operator on restricted Schur polynomials labeled by Young diagrams with p long columns or p long rows. A new version of Schur-Weyl duality provides a powerful approach to the computation and manipulation of the symmetric group operators appearing in the restricted Schur polynomials. Using this new technology, we are able to evaluate the action of the one loop dilatation operator. The result has a direct and natural connection to the Gauss Law constraint for branes with a compact world volume. Generalzing previous results, we find considerable evidence that the dilatation operator reduces to a decoupled set of harmonic oscillators. This strongly suggests that integrability in N = 4 super Yang-Mills theory is not just a feature of the planar limit, but extends to other large N but non-planar limits.