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Semilattices and indecomposable elements
Kuděj, Martin ; Kala, Vítězslav (advisor) ; Korbelář, Miroslav (referee)
This thesis concerns the theory of semilattices, which are non-trivial discrete additive submonoids of Rn , which are contained in a cone. Special emphasis is on their indecomposable elements. The most important example of semilattices is derived from real quadratic number fields, which involves the most parts of the thesis and all indecomposable elements of such semilattices are characterised in two ways. That includes using various tools from number theory, mainly con- tinued fractions, their corresponding semiconvergents and their approximation properties, Farey pairs, but also some tools from algebraic number theory. The final part of the thesis concerns the upper bound of the norm of indecompos- able elements in a semilattice, derived from the Minkowski embedding of the corresponding number field. 1
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Constructions of Commutative Semirings and Radical Rings
Korbelář, Miroslav ; Kepka, Tomáš (advisor) ; Němec, Petr (referee) ; Příhoda, Pavel (referee)
In this dissertation we deal with constructive methods applied to the commutative semirings and commutative radical rings. In Chapter 2 we study the class S of the commutative subdirectly irreducible radical rings. We present a few constructive methods for them and using the reflection of the category of the commutative rings into the category of the commutative radical rings we derive a lot of examples of rings in S with various properties. We prove that a ring S 2 S is noetherian if and only if it is finite. We show partial results in the classification of factors of S modulo monoliths. In Chapter 3 we introduce, using the p-prime valuation for all primes p, a set of characteristic sequences that can be assign to every subsemiring of Q+. We find and classify all maximal subsemirings of positive rational numbers and show that every proper subsemiring of Q+ is contained in at least one of them. This results was published in [16]. In Chapter 4 we construct, using the approach from the Chapter 4, a new large subclass of the class CongSimp of all proper congruence-simple subsemirings of Q+, classify all the maximal elements of CongSimp and show that every element of CongSimp is contained in at least one of them. In Chapter 5 we find an equivalent condition under which is the semiring Q+[ ] C, 2 C, contained in...
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Finitely generated semirings and semifields
Šíma, Lucien ; Kala, Vítězslav (advisor) ; Korbelář, Miroslav (referee)
We investigate commutative semirings, which are formed by a ground set equipped with two binary associative and commutative operations such that one distributes over the other. We narrow down our interest to ideal-simple semirings, that is, semirings without proper ideals. We present the classification of ideal-simple semirings and deal with some classes of ideal-simple semirings, namely semifields and parasemifields. The main result of this thesis is giving tight bounds on the minimal number of generators needed to generate a parasemifield as a semiring. We also study how the semifields that are finitely generated as a semiring look like. Last, but not least, we show that every finitely generated ideal-simple semiring is finitely-generated as a multiplicative semigroup.
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Cryptography based on semirings
Mach, Martin ; Korbelář, Miroslav (advisor) ; El Bashir, Robert (referee)
Cryptography based on semirings can be one of the possible approaches for the post-quantum cryptography in the public-key schemes. In our work, we are interested in only one concrete semiring - tropical algebra. We are examining one concrete scheme for the key-agreement protocol - tropical Stickel's protocol. Although there was introduced an attack on it, we have implemented this attack and more importantly, stated its complexity. Further, we propose other variants of Stickel's protocol and we are investigating their potential for practical usage. During the process, we came across the theory of tropical matrix powers, thus we want to make an overview of it due to the use in cryptography based on matrices over the tropical algebra semiring. 1
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NTRU cryptosystem and its modifications
Poláková, Kristýna ; Příhoda, Pavel (advisor) ; Korbelář, Miroslav (referee)
The theses firstly introduces the basics of lattice problems. Then it focuses on various aspects of the cryptosystem NTRU which is based on the mentioned problems. The system is then compared with the most common encryption methods used nowadays. Its supposed quantum resistence is mentioned briefly. Subsequently the author tries to minimize the system's disadvantages by various cryptosystem modifications. Powered by TCPDF (www.tcpdf.org)
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Constructions of Commutative Semirings and Radical Rings
Korbelář, Miroslav ; Kepka, Tomáš (advisor) ; Němec, Petr (referee) ; Příhoda, Pavel (referee)
In this dissertation we deal with constructive methods applied to the commutative semirings and commutative radical rings. In Chapter 2 we study the class S of the commutative subdirectly irreducible radical rings. We present a few constructive methods for them and using the reflection of the category of the commutative rings into the category of the commutative radical rings we derive a lot of examples of rings in S with various properties. We prove that a ring S 2 S is noetherian if and only if it is finite. We show partial results in the classification of factors of S modulo monoliths. In Chapter 3 we introduce, using the p-prime valuation for all primes p, a set of characteristic sequences that can be assign to every subsemiring of Q+. We find and classify all maximal subsemirings of positive rational numbers and show that every proper subsemiring of Q+ is contained in at least one of them. This results was published in [16]. In Chapter 4 we construct, using the approach from the Chapter 4, a new large subclass of the class CongSimp of all proper congruence-simple subsemirings of Q+, classify all the maximal elements of CongSimp and show that every element of CongSimp is contained in at least one of them. In Chapter 5 we find an equivalent condition under which is the semiring Q+[ ] C, 2 C, contained in...
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