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Let L/F be a finite separable extension. L* = L\{0}, and T(L*/F*) be the torsion subgroup of L*/F*. We explicitly determined T(L*/F*) when L/F is an abelian extension. This information is used to study the structure of T(L*/F*). In particular T(F(α)*/F*) when αᵐ = a ∈ F is explicitly determined. Let Xᵐ - a be irreducible over F with char F χ m and let α be a root of Xᵐ - a. We study the lattice of subfields of F(α)/F and to this end C(F(α)/F,k) is defined to be the number of subfields of F(α) of degree k over F. C(f(α)/F,pⁿ) is explicitly determined for p a prime and the following structure theorem for the lattice of subfields is proved. Let N be the maximal normal subfield of F(α) and set n = [N:F], then C(F(α)/F,k) = C(F(α)/F,(k,n)) = C(N/F,(k,n)). The irreducible binomials X⁸ - b, X⁸ - c are said be equivalent if there exist roots β⁸ = b, γ⁸ = c that F(β) = F(γ). All the mutually inequivalent binomials which have roots in F(α) are determined. These results are applied the study of normal binomials and those irreducible binomials X²ᵉ - a which are normal over F(charF ≠ 2) together their Galois groups are characterized. We finished by considering the radical extension F(α)/F, αᵐ ∈ F, where the binominal Xᵐ - αᵐ is not necessarily irreducible. We see that in the case not every subfield of F(α)/F is the compositum of subfields of prime power order. We determine some conditions such that if F ⊆ H ⊆ F(α) with [H:F] = pᵘq, p a prime, (p,q) = 1, then there exists a subfield F ⊆ R ⊆ H where [R:F] = pᵘ.

In this thesis we are concerned with arithmetic in a certain partially ordered, commutative semigroup D. The first chapter investigates the class of countable Boolean algebras from which this semigroup arises. The elements of D correspond to the isomorphism classes of the Boolean algebras under consideration. In Chapter 2 we begin the study of the semigroup structure of D. D is axiomatically described by three groups of axioms. It is proved that these axioms are categorical. The ordering of D is used to investigate the multiplication. The set of T of torsion elements of D (elements with only finite many distinct powers), form a subsemigroup whose structure is studied. There is a natural torsion free quotient D/T whose structure is also investigated. In Chapter 3, the axioms are used to characterize elements s of T in terms of the arithmetic in the subsemigroup generated by the elements that are smaller than s. The characterization is used to determine elements of T that cover a single element. In the last part of Chapter 3, we obtain some sufficient, purely combinatorial conditions for an element to have infinite order.

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