## Tohoku Mathematical Journal 2016 December SECOND SERIES VOL. 68, NO. 4

 Tohoku Math. J. 68 (2016), 515-558

Title THE EQUIVARIANT $K$-THEORY AND COBORDISM RINGS OF DIVISIVE WEIGHTED PROJECTIVE SPACES

Author Andrea Cattaneo and Alice Garbagnati

(Received April 9, 2014, revised February 17, 2015)
Abstract. We consider Calabi--Yau 3-folds of Borcea--Voisin type, i.e. Calabi--Yau 3-folds obtained as crepant resolutions of a quotient $(S\times E)/(\alpha_S\times \alpha_E)$, where $S$ is a K3 surface, $E$ is an elliptic curve, $\alpha_S\in \operatorname{Aut}(S)$ and $\alpha_E\in \operatorname{Aut}(E)$ act on the period of $S$ and $E$ respectively with order $n=2,3,4,6$. The case $n=2$ is very classical, the case $n=3$ was recently studied by Rohde, the other cases are less known. First, we construct explicitly a crepant resolution, $X$, of $(S\times E)/(\alpha_S\times \alpha_E)$ and we compute its Hodge numbers; some pairs of Hodge numbers we found are new. Then, we discuss the presence of maximal automorphisms and of a point with maximal unipotent monodromy for the family of $X$. Finally, we describe the map $\mathcal{E}_n: X \rightarrow S/\alpha_S$ whose generic fiber is isomorphic to $E$.

Mathematics Subject Classification. Primary 14J32; Secondary 14J28, 14J50.

Key words and phrases. Calabi--Yau 3-folds, automorphisms, K3 surfaces, elliptic fibrations, isotrivial fibrations.

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