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The Łojasiewicz gradient inequality in a neighbourhood of the fibre. (English) Zbl 1108.14051

The author studies the Łojasiewicz gradient exponent at infinity for polynomials in two variables. First he obtains an effective estimate for critical values of a polynomial in one variable [cf. S. Smale, Bull. Am. Math. Soc., New Ser. 4, 1–36 (1981; Zbl 0456.12012)] and generalizes the Bochnak-Łojasiewicz inequality from S. Spodzieja [Bull. Pol. Acad. Sci., Math. 50, No. 3, 273–281 (2002; Zbl 1057.14076)] as follows: for a polynomial map \(f \colon \mathbb C^n \rightarrow \mathbb C\) there are constants \(C, \varepsilon > 0\) such that the condition \(| f(z)| \leq \varepsilon\) provides \(| z| |\text{grad}\, f(z)| \geq C| f(z)|.\) Then he considers a polynomial in two variables of degree \(d >2\) such that \(0\in \mathbb C\) is a bifurcation point of \(f\) at infinity and proves that for any neighbourhood of the fiber \(f^{-1}(0)\) the gradient exponent of \(f\) does not exceed \((-1-1/(d-2)).\) In particular, this implies a sharper version of Malgrange condition \((m)\) [A.Parusinski, Compos. Math. 97, No. 3, 369–384 (1995; Zbl 0840.32007)]. The author also proves the following separation condition for a polynomial in two variables and its gradient: \(| f(z)| \leq \varepsilon \Rightarrow |\text{grad}\, f(z)| \geq C| f(z)|^q\) for suitable \(C, \varepsilon, q >0.\)

MSC:

14R25 Affine fibrations
58K05 Critical points of functions and mappings on manifolds
57R70 Critical points and critical submanifolds in differential topology
32S05 Local complex singularities
14B05 Singularities in algebraic geometry
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