|dc.description.abstract||The thesis consists of four chapters. First chapter is introductory. In Chapter 2, we recall some basic facts from the singularity theory of algebraic varieties (see Section
2.2) and the theory of minimal models (see Section 2.3), which will be used throughout the rest of
the thesis. We also make some conventions on the notions and notation used in the thesis (see
Each Chapter 3 and 4 starts with some preliminary results (see Sections 3.1 and 4.1, respectively).
Each Chapter 3 and 4 ends with some corollaries and conclusive remarks (see Sections 3.7 and 4.4, respectively).
In Chapter 3, we prove Theorem 1.2.7, providing the complete description of Halphen pencils on a
smooth projective quartic threefold X in P4. Let M be such a pencil. Firstly, we show that M ⊂ |
− nKX | for some n ∈ N, and
the pair (X,1n M) is canonical but not terminal. Further, if the set of not terminal centers CS(X, 1 ) (see Remark 2.2.8) does not contain points, we show that n = 1 (see Section 3.2). Finally, if there is a point P ∈ CS(X, n M), in Section 3.1 we show first that a general M ∈ M has multiplicity 2n at P (cf. Example 1.2.3). After that, analyzing the shape of the Hessian of the equation of X at
the point P , we prove that n = 2 and M coincides with the exceptional Halphen pencil from Example
1.2.6 (see Sections 3.3–3.6). In Chapter 4, we prove Theorem 1.2.11, which shows, in particular, that a general smooth K3 surfaces of type R is an anticanonical section of the Fano threefold X with canonical Gorenstein singularities and genus 36. In Section 4.2, we prove that X is unique up to an isomorphism and has
a unique singular point, providing the geometric quotient construction of the moduli
space F in Section 4.3 (cf. Remark 1.2.12). Finally, in Section 4.3 we prove that the forgetful map F −→ KR is generically surjective.||en