| 1. |
- Xia, Jiacheng, 1991
(författare)
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Vector-valued Eisenstein series of congruence types and their products
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Historically, Kohnen and Zagier connected modular forms with period polynomials, and as a consequence of this association concluded that the products of at most two Eisenstein series span all spaces of classical modular forms of level 1. Later Borisov and Gunnells among other authors extended the result to higher levels. We consider this problem for vector-valued modular forms, establish the framework of congruence types and obtain the structure of the space of vector-valued Eisenstein series using tools from representation theory. Based on this development and historic results, we show that the space of vector-valued modular forms of certain weights and any congruence type can be spanned by the invariant vectors of that type tensor at most two Eisenstein series.
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| 3. |
- Karlsson, Carl-Joar, 1995
(författare)
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Game theory and applications
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Individual-level interactions and decisions spread though populations and change the collective-level dynamics in an intricate way. Nevertheless, game theory is well suited for the unification of these viewpoints. This thesis introduces the works The strength of diversity and Decisions and disease: a mechanism for the evolution of cooperation, which show that games have broad applications in population dynamics modeling. In The strength of diversity we calculate the equilibrium strategies of the so-called game of teams. The game of teams is an individual-level competition between teams, and a team's strategy in this context is a distribution of strength over the team members. It turns out that the equilibrium strategies are flat distributions or 'alternating' flat distributions whenever there exists equilibrium strategies. In Decisions and disease: a mechanism for the evolution of cooperation we combine the classic SIR and SIS models from epidemiology with the prisoner's dilemma game. The transmission rate is computed as the average over defecting and cooperating individuals, and the individuals are subjected to a replicator equation that takes into account the portion of infectious members of the population. We compute the steady state solutions and interpret the results.
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| 5. |
- Larsson, Joel, 1987-
(författare)
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On random cover and matching problems
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of the following papers.I J. Larsson, The Minimum Matching in Pseudo-dimension 0 < q < 1, submittedII V. Falgas-Ravry, J. Larsson, K. Markström, Speed and concentration of the covering time for structured coupon collectors, submittedIII J. Larsson, K. Markström, Biased random k-SAT problems, manuscriptThese papers can all be seen as variations on the same question: Given a large set V and a family F of subsets of V, each assigned a (random) weight, we assign each subfamily G ⊆ F a cost based on the weights of sets that occur in it. What will the minimal cost of a subfamiliy G that covers V be?In the first paper, we search for a disjoint cover of the ground set V = {u_1,u_2,...u_n,v_1,v_2,...v_n}, using random 2-sets of the form {u_i, v_j}. In other words, we search for matchings in a bipartite graph. Each edge receives a random weight distributed uniformly in [0, 1], and the cost of a perfect matching using edges with weights l_1,l_2,...l_n is Σ_{i=1}^n l_i^{1/q} for some q > 0.The second paper lives in a more general setting. There we search for any cover of the ground set V, for general families F. Each set f ∈ F receives weight w(f) uniformly at random from [0,1]. The cost of a cover f_1,f_2,...f_m is then taken to be max_i w(f_i). This is equivalent (after a rescaling) to drawing sets from F at Poisson times, and the cost of a cover is the first time V is covered. This problem is known under a number of names, perhaps most famously the coupon collector problem. In the classical formulation, single elements of V are drawn, not sets. The classical coupon collector thus corresponds to the family F consisting of singleton sets, and we call the version allowing larger sets structured coupon collector problems. The main concern of this paper is to identify relevant properties of F that affect the covering time (i.e. minimal cost of a cover), and to provide (easily checkable) sufficient conditions for concentration of the covering time.For the third paper we narrow the scopes once more, and study the biased random k-SAT problem. The random k-SAT problem can be seen as a special case of the structured coupon collector, but a special case that has far richer structure than the generic case. The ground set is the hypercube Σ_n = {0, 1}^n, and the coupons are all the k-codimensional subcubes of Σ_n. We study a slight variation on this problem: subcubes are drawn with a constant bias towards 0, so that vertices in Σ_n with fewer 1's and more 0's are easier to cover.
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| 6. |
- Pham, Lan Anh
(författare)
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On avoiding and completing edge colorings
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- These papers are all related to the problem of avoiding and completing an edge precoloring of a graph. In more detail, given a graph G and a partial proper edge precoloring φ of G and a list assignment L for every non-colored edge of G, can we extend the precoloring to a proper edge coloring avoiding any list assignment? In the first paper, G is a d-dimensional hypercube graph Qd, a partial proper edge precoloring φ and every list assignment L must satisfy certain sparsity conditions. The second paper still deals with d-dimensional hypercube graph Qd, but the list assignment L for every edge of Qd is an empty set and φ must be a partial proper edge precoloring of at most (d - 1) edges. For the third paper, G can be seen as a complete 3-uniform 3-partite hypergraph, every list assignment L must satisfy certain sparsity conditions but we do not have a partial proper edge precoloring φ on edges of G.
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| 7. |
- Potka, Samu, 1991-
(författare)
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Limit shapes of standard Young tableaux and sorting networks via the Edelman-Greene correspondence
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of the following two articles.New properties of the Edelman–Greene bijection. Edelman and Greene constructed a correspondence between reduced words of the reverse permutation and standard Young tableaux. We prove that for any reduced word the shape of the region of the insertion tableau containing the smallest possible entries evolves exactly as the upper-left component of the permutation’s (Rothe) diagram. Properties of the Edelman–Greene bijection restricted to 132-avoiding and 2143-avoiding permutations are presented. We also consider the Edelman-Greene bijection applied to non-reduced words.On random shifted standard Young tableaux and 132-avoiding sorting networks. We study shifted standard Young tableaux (SYT). The limiting surface of uniformly random shifted SYT of staircase shape is determined, with the integers in the SYT as heights. This implies via properties of the Edelman–Greene bijection results about random 132-avoiding sorting networks, including limit shapes for trajectories and intermediate permutations. Moreover, the expected number of adjacencies in SYT is considered. It is shown that on average each row and each column of a shifted SYT of staircase shape contains precisely one adjacency.
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| 8. |
- Ruoyu, Wang, 1987-
(författare)
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A tale of trees and leaves : subtrees and local convergence
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Given a tree T, a subtree in T is a subgraph that is a tree itself. The set of subtrees in a tree is related to many important graph parameters, one of which is the mean subtree order. Jamison initiated and laid the groundwork of the study of mean subtree order in the early 1980s and raised in total seven conjectures and open questions, on which the first two projects in the licentiate are based. In the first project, we will give a descrip- tion of the subtrees that achieve maximal/minimal local mean among all the subtrees of the same order. In the second project, we provide a proof that the mean subtree order decreases at least 1/3 in contracting an edge of a tree, which closed one of two conjectures that had remained open. Lastly, we study the asymptotic behavior of the number of subtrees for a sequence of trees that converges in the Benjamini–Schramm sense.
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| 9. |
- Thapper, Johan, 1977-
(författare)
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Combinatorial Considerations on Two Models from Statistical Mechanics
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interactions between combinatorics and statistical mechanics have provided many fruitful insights in both fields. A compelling example is Kuperberg’s solution to the alternating sign matrix conjecture, and its following generalisations. In this thesis we investigate two models from statistical mechanics which have received attention in recent years.The first is the fully packed loop model. A conjecture from 2001 by Razumov and Stroganov opened the field for a large ongoing investigation of the O(1) loop model and its connections to a refinement of the fully packed loop model. We apply a combinatorial bijection originally found by de Gier to an older conjecture made by Propp.The second model is the hard particle model. Recent discoveries by Fendley et al. and results by Jonsson suggests that the hard square model with cylindrical boundary conditions possess some beautiful combinatorial properties. We apply both topological and purely combinatorial methods to related independence complexes to try and gain a better understanding of this model.
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| 10. |
- Gustafsson, Henrik, 1988
(författare)
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Automorphic string amplitudes
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the non-perturbative properties of higher derivative interactions appearing in the low-energy expansion of four-graviton scattering amplitudes in toroidal compactifications of type IIB string theory. We summarise the arguments for finding such higher derivative corrections in terms of automorphic forms using U-duality, supersymmetry and string perturbation theory. The perturbative and non-perturbative parts can then be studied from their Fourier expansions. To be able to compute such Fourier coefficients we use the adelic framework as an intermediate step which also gives a new perspective on the arithmetic content of the scattering amplitudes.We give a review of known methods for computing certain classes of Fourier coefficients from the mathematical literature as presented in Paper I of the appended papers, and of our own work in Paper II towards computing some of the remaining coefficients of interest in string theory.
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