Some extremal properties of positive trigonometric polynomials

For n=8 an upper bound is given for the functional $$V_n = \mathop {\inf }\limits_{t_n } \frac{{\alpha _1 + \alpha _2 + \cdots + \alpha _n }}{{\left( {\sqrt {\alpha _1 } - \sqrt {\alpha _0 } } \right)^2 }}$$ , which is defined on the class of even, nonnegative, trigonometric polynomials \(t_n (\phi ) = \sum\nolimits_{k = 0}^n {\alpha _k } cos k\phi \) , such that α _k ? 0 (k=0, ...,n) α₁>α₀ :V _s ? 34.54461566.     

Some extremal problems for trigonometric polynomials

Three extremal problems for trigonometric polynomials are studied in this paper. The first was initiated by Maiorov. It relates to the trigonometric polynomials with n nonzero harmonics. The L_p-norm of the Weyl derivative is compared with the L_q-norm of the polynomial. The other two problems have appeared in the recent paper by Oswald. They deal with the polynomials of degree n. The minimum of L_p-norm with respect to the choice of phases is compared with l_q-norm of its coefficients.     

Some extremal properties of orthogonal polynomials

Using a particular way of normalizing the orthogonal polynomials, which is most commonly encountered in the synthesis of filtering networks in communication and electronic engineering, two theorems concerning the extremal properties of orthogonal polynomials are first proved. The results are then applied to find the minimum value and the minimizing function for various definite integrals involving weight functions of classical orthogonal polynomials.     

An extremal problem for trigonometric polynomials

Let be a trigonometric polynomial of degree The problem of finding the largest value for in the inequality is studied. We find exactly provided is the conjugate of an even integer and For general we get an interval estimate for where the interval length tends to as tends to

     

Some extremal properties of the integral of legendre polynomials

1. Introduction and Main ResultsDenote by Pn the set of algebraic polynomials of degree not exceeding n. LetX = {X = (xl,xz,...,x.). 1 = xl > xz >' > xtL--l > xtL = --l}, n 2 2and let for X E X.Erd6s in [21 raised the question of determining Y E X such thatReceived July 21, 1999.also conjectured that Y = Z satisfyingw,,(Z, x) = c1' l" Pt.--1 (x) dx,j-- 1 Pt.-- 1 (x) dx,1 .2>where P,--1 stands for the (n -- 1)th Legendre polynomial normalized by Pn--1 (1) = l. Thiscolljecture was di…     

Some Bernstein-type inequalities for trigonometric polynomials

The refinements of some well-known Bemstein-type inequalities for trigonometric polynomials are obtained.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 45, No. 3, pp. 428–430, March, 1993.     

Two extremal problems for trigonometric polynomials with a prescribed number of harmonics

Translated from Matematicheskie Zametki, Vol. 49, No. 1, pp. 12–18, January, 1991.     

Some extremal problems for polynomials in integral metrics

Translated fromMatematicheskie Zametki, Vol. 58, No. 6, pp. 940–941, December, 1995.     

Conjugate trigonometric polynomials

Translated from Matematicheskie Zametki, Vol. 48, No. 4, pp. 110–114, October, 1990.     

Some extremal problems for algebraic polynomials in loaded spaces

In a loaded Jacobi space with the inner product

Local properties of functions and approximation by trigonometric polynomials

Suppose Φ_{p, E} (p>0 an integer, E ?[0, 2π]) is a family of positive nondecreasing functions? _x(t) (t>0, x∈ E) such that? _x(nt)≤n^P ? _x(t) (n=0,1,...), t_n is a trigonometric polynomial of order at most n, and Δ _h ^l (f, x) (l>0 an integer) is the finite difference of orderl with step h of the functionf.THEOREM. Supposef (x) is a function which is measurable, finite almost everywhere on [0, 2π], and integrable in some neighborhood of each point xε E,? _X εΦ_p,E and $$\overline {\mathop {\lim }\limits_{\delta \to \infty } } |(2\delta )^{ - 1} \smallint _{ - \delta }^\delta \Delta _u^l (f,x)du|\varphi _x^{ - 1} (\delta ) \leqslant C(x)< \infty (x \in E).$$ . Then there exists a sequence {t _n } _n=1 ^∞ which converges tof (x) almost everywhere, such that for x ε E $$\overline {\mathop {\lim }\limits_{n \to \infty } } |f(x) - l_n (x)|\varphi _x^{ - 1} (l/n) \leqslant AC(x),$$ where A depends on p andl.     

On extremal properties for the polar derivative of polynomials

If p(z) is a polynomial of degree n having all its zeros on |z| = k, k ≤ 1, then it is proved[5] that max |z|=1 |p′(z)| ≤ kn1n + kn m|z|=ax1 |p(z)|. In this paper, we generalize the above inequality by extending it to the polar derivative of a polynomial of the type p(z) = cnzn + ∑n j=μ cn jzn j, 1 ≤μ≤ n. We also obtain certain new inequalities concerning the maximum modulus of a polynomial with restricted zeros.     

Extremal properties of certain trigonometric functions and Chebyshev polynomials

For a wide class of symmetric trigonometric polynomials, the minimal deviation property is established. As a corollary, the extremal property is proved for the components of the Chebyshev polynomial mappings corresponding to symmetric algebras A _α.