We study the supremum of some random Dirichlet polynomials $D_N\left(t\right)={\sum }_{n=2}^N\epsilon ₙdₙn^{-\sigma -it}$, where (εₙ) is a sequence of independent Rademacher random variables, the weights (dₙ) are multiplicative and 0 ≤ σ < 1/2. Particular attention is given to the polynomials ${\sum }_{n{\in }_{\tau }}\epsilon ₙn^{-\sigma -it}$, ${}_{\tau }=2\le n\le N:P⁺\left(n\right)\le p_{\tau }$, P⁺(n) being the largest prime divisor of n. We obtain sharp upper and lower bounds for the supremum expectation that extend the optimal estimate of Halász-Queffélec, $su{p}_{t\in ℝ}\left|{\sum }_{n=2}^N\epsilon ₙn^{-\sigma -it}\right|\approx \left(N^{1-\sigma }\right)/\left(logN\right)$. The proofs are entirely based on methods of stochastic processes, in particular the metric...

The paper has been presented at the 12th International Conference on Applications of Computer Algebra, Varna, Bulgaria, June, 2006.To get guaranteed machine enclosures of a special function f(x), an upper bound ε(f) of the relative error is needed, where ε(f) itself depends on the error bounds ε(app); ε(eval) of the approximation and evaluation error respectively. The approximation function g(x) ≈ f(x) is a rational function (Remez algorithm), and with sufficiently high polynomial degrees ε(app) becomes...

L’article donne des réponses optimales ou presque optimales aux questions suivantes, qui remontent à Stieltjes, Landau et Bohr, et concernent des séries de Dirichlet $A_j=\sum _{n=1}^{\infty }a(j,n)n^{-s}$$(j=1,2...$

We characterize the power series ${\sum }_{n=0}^{\infty }{c}_n{z}^n$ with the geometric property that, for sufficiently many points $z$, $\left|z\right|=1$, a circle $C\left(z\right)$ contains infinitely many partial sums. We show that ${\sum }_{n=0}^{\infty }{c}_n{z}^n$ is a rational function of special type; more precisely, there are $t\in ℝ$ and $n_0$, such that, the sequence $c_n{e}^{\mathrm{int}}$, $n\ge n_0$, is periodic. This result answers in the affirmative a question of J.-P. Kahane and furnishes stronger versions of the main results of [Katsoprinakis, Arkiv for Matematik]. We are led to consider special families of circles $C\left(z\right)$ with...

We give another version of the recently developed abstract theory of universal series to exhibit a necessary and sufficient condition of polynomial approximation type for the existence of universal elements. This certainly covers the case of simultaneous approximation with a sequence of continuous linear mappings. In the case of a sequence of unbounded operators the same condition ensures existence and density of universal elements. Several known results, stronger statements or new results can be...

Firstly, we give extensions of results of Gelfand, Esterle and Katznelson--Tzafriri on power-bounded operators. Secondly, some results and questions relating to power-bounded elements in the unitization of a commutative radical Banach algebra are discussed.

Cet exposé est une introduction au calcul étranger d’Écalle, c’est-à-dire au calcul des obstructions à la sommabilité de Borel d’une grande classe de séries formelles, les fonctions résurgentes d’Écalle. La théorie d’Écalle éclaire d’un jour neuf le célèbre phénomène de Stokes qui est illustré ici dans le contexte de la méthode du col.