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This paper concerns the construction of minimal varieties with small canonical volumes. The first part devotes to establishing an effective nefness criterion for the canonical divisor of a weighted blow-up over a weighted hypersurface, from which we construct plenty of new minimal $3$-folds including $59$ families of minimal $3$-folds of general type, several infinite series of minimal $3$-folds of Kodaira dimension $2$, $2$ families of minimal $3$-folds of general type on the Noether line, and $12$ families of minimal $3$-folds of general type near the Noether line. In the second part, we prove effective lower bounds of canonical volumes of minimal $n$-folds of general type with canonical dimension $n-1$ or $n-2$. Examples are provided to show that the theoretical lower bounds are optimal in dimension less than or equal to $5$ and nearly optimal in higher dimensions., Comment: 40 pages, all examples are manually verified; v2: final version to appear in J. Differential Geom

We study the deformation of $G$-marked stable curves in the case where $G$ is a cyclic group, and construct a parameterizing space for $G$-marked stable curves of a given numerical type. This is then used in order to study the components of the locus of stable curves admitting the action of a cyclic group of non prime order $d$, extending work of F. Catanese in the case where $d$ is prime., Comment: 14 pages

The main goal of this paper is to show that Castelnuovo- Enriques' $P_{12}$-theorem also holds for algebraic surfaces $S$ defined over an algebraically closed field $k$ of positive characteristic ($char(k) = p > 0$). The $P_{12}$-theorem is a precise version of the rough classification of algebraic surfaces, in particular the conditions $P_{12} = 0$, $P_{12} = 1$,$P_{12} \geq 2$ are respectively equivalent to : Kodaira dimension $-\infty, 0 , \geq 1$. The result relies on a main theorem describing the growth of the plurigenera for properly-elliptic or properly quasi-elliptic surfaces (surfaces with Kodaira dimension equal to 1). We also discuss the limit cases, i.e. the families of surfaces which show that the results of the main theorem are sharp., Comment: 19 pages; added some remark, for instance that for complex non algebraic surfaces one needs the plurigenus $P_{42}$

The aim of this paper is to investigate the closed subschemes of moduli spaces corresponding to projective varieties which admit an effective action by a given finite group $G$. To achieve this, we introduce the moduli functor $\mathsf{M}^G_h$ of $G$-marked Gorenstein canonical models with Hilbert polynomial $h$, and prove the existence of $\mathfrak{M}_h[G]$, the coarse moduli scheme for $\mathsf{M}^G_h$. Then we show that $\mathfrak{M}_h[G]$ has a proper and finite morphism onto $\mathfrak{M}_h$ so that its image $\mathfrak{M}_h(G)$ is a closed subscheme. In the end we obtain the canonical representation type decomposition $\mathcal{D}_h[G]$ of $\mathfrak{M}_h[G]$ and use $\mathcal{D}_h[G]$ to study the structure of $\mathfrak{M}_h[G]$., Comment: 25 pages, minor changes, a new section (section 5) containing new results has been added

The aim of this paper is to determine the irreducible components of M_g(D_n), the locus inside M_g of the curves admitting an effective action by the dihedral group D_n. This is done by classifying pairs (H,H') of distinct subgroups of the mapping class group Map_g, such that both H and H' are isomorphic to D_n and the fixed point locus of H inside the Teichm\"uller space T_g is contained in the fixed point locus of H'., Comment: 23 pages, 12 tables, minor changes, to be published in RENDICONTI DEL CIRCOLO MATEMATICO DI PALERMO