Mori dreamness of blowups of weighted projective planes

We consider the blowup X(a, b, c) of a weighted projective space $${\mathbb {P}}(a,b,c)$$ P ( a , b , c ) at a general nonsingular point. We give a sufficient condition for a curve to be a negative curve on X(a, b, c) in terms of $$\chi ({\mathcal {O}}_X(C))$$ χ ( O X ( C ) ) . This can be applied to find the effective cone of X(a, b, c) and can serve as a starting point to prove the Mori dreamness of blowups of many weighted projective planes. We confirm the Mori dreamness of some X(a, b, c) as examples of our method.

Equations and Complexity for the Dubois–Efroymson Dimension Theorem

LetRbe a real closed field, letX⊂Rnbe an irreducible real algebraic set and letZbe an algebraic subset ofXof codimension ≥ 2. Dubois and Efroymson proved the existence of an irreducible algebraic subset ofXof codimension 1 containingZ. We improve this dimension theorem as follows. Indicate by μ the minimum integer such that the ideal of polynomials inR[x1, … ,xn] vanishing onZcan be generated by polynomials of degree ≤ μ. We prove the following two results: (1) There exists a polynomialP∈R[x1, … ,xn] of degree≤ μ+1 such thatX∩P–1(0) is an irreducible algebraic subset ofXof codimension 1 containingZ. (2) LetFbe a polynomial inR[x1, … ,xn] of degreedvanishing onZ. Suppose there exists a nonsingular pointxofXsuch thatF(x) = 0 and the differential atxof the restriction ofFtoXis nonzero. Then there exists a polynomialG∈R[x1, … ,xn] of degree ≤ max﹛d, μ + 1﹜ such that, for eacht∈ (–1, 1) \ ﹛0﹜, the set ﹛x∈X|F(x) +tG(x) = 0﹜ is an irreducible algebraic subset ofXof codimension 1 containingZ. Result (1) and a slightly different version of result (2) are valid over any algebraically closed field also.

A Local Ergodic Theorem for Multiparameter Superadditive Processes

In this paper a local ergodic theorem is proved for positive (multiparameter) superadditive processes with respect to (multiparameter) semiflows of nonsingular point transformations on a a-finite measure space. The theorem obtained here generalizes Akcoglu-Krengel's [2] local ergodic theorem for superadditive processes with respect to semiflows of measure preserving transformations. The proof is a refinement of Akcoglu-Krengel's argument in [2]. Also, ideas of Feyel [3] and the author [4], [5] are used.