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Fractional indices of log Del Pezzo surfaces. (English. Russian original) Zbl 0724.14023
Math. USSR, Izv. 33, No. 3, 613-629 (1989); translation from Izv. Akad. Nauk SSSR, Ser. Mat. 52, No. 6, 1288-1304 (1988).
Let X be a normal projective complex surface having at most log-terminal singularities and such that the anti-canonical class $$-K_ X$$ is ample (meaning that $$-mK_ X$$ becomes an ample Cartier divisor for some integer $$m\geq 1)$$. Such a surface will be referred to as a log Del Pezzo surface. For a log Del Pezzo surface X one can define the fractional index r(X) of X as the (positive) rational number $$r=r(X)$$ such that $$- K_ X=rH$$, where H is an ample Cartier divisor primitive in Pic(X). The paper under review describes the set $$R=\{r(X)| \quad X\quad a\quad \log Del Pezzo\quad surface\}$$ by proving (among other things) the following theorem:
The set R has the following accumulation points: 0 and 1/m for any natural number m. All of these points are limit points from above and not from below. Moreover, for any natural number m, one can choose a sufficiently small punctured neighbourhood $$Q_ m=\{x\in {\mathbb{R}}| \quad 0<| x-1/m| <\epsilon_ m\}$$ in such a way that all log Del Pezzo surfaces X with $$r(X)\in Q_ m$$ can be classified explicitly.
[See also the correction of this paper announced below, ibid. 54, No.5, 1112 (1990).]

##### MSC:
 14J25 Special surfaces 14J17 Singularities of surfaces or higher-dimensional varieties 14C20 Divisors, linear systems, invertible sheaves
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