Generalizations of Zygmund-type integral inequalities for the polar derivative of a complex polynomial

Some Zygmund-type integral inequalities for the polar derivatives of complex polynomials, inspired by the classical Bernstein-type inequalities that relate the uniform norms of polynomials and their derivatives on the unit circle, are investigated. The obtained results sharpen as well as generalize some already known Lδ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$L^{\delta }$\end{document}-estimates between polynomials and their polar derivatives.


Introduction and preliminaries
Let P(z) := n ν=0 a ν z ν be a complex polynomial of degree n and P (z) be its derivative. There is a well-known classical Bernstein result for two polynomials P(z) and Q(z) with degree of P(z) not exceeding that of Q(z) and Q(z) = 0 for |z| > 1. Namely, the inequality |P(z)| ≤ |Q(z)| on the unit circle |z| = 1 implies the inequality of their derivatives |P (z)| ≤ |Q (z)| on |z| = 1. In particular, this result allows one to establish the famous Bernstein inequality [3] in the uniform norm on the unit circle, i.e., if P(z) is a polynomial of degree n, it is true that max |z|=1 P (z) ≤ n max |z|=1 P(z) . (1.1) This inequality (1.1) was proved by Bernstein in 1912, and is best possible with equality holding for monomials P(z) = cz n , where c is an arbitrary complex number. If we restrict ourselves to the class of polynomials having no zeros in |z| < 1, then inequality (1.1) can be sharpened. In fact, Erdős conjectured and later Lax [8] proved that, if P(z) = 0 in |z| < 1, In 1969 Malik [9] proved an extension of (1.2) under the condition that P(z) = 0 in |z| < k, where k ≥ 1, i.e., max |z|=1 P (z) ≤ n 1 + k max |z|=1 P(z) . (1.3) This inequality was generalized in different directions. Chan and Malik [4] proved that, if P(z) = a 0 + n j=μ a j z j , μ ≥ 1, is a lacunary polynomial of degree n and P(z) = 0 in |z| < k, k ≥ 1, then (1.4) Inequality (1.4) was independently proved by Qazi (see [17], Lemma 1), who also under the same conditions proved that For a polynomial P(z) of degree n, now we define the so-called the polar derivative of P(z) with respect to the point α as This polynomial D α P(z) is of degree at most n -1 and it generalizes the ordinary derivative P (z) in the following sense: uniformly with respect to z for |z| ≤ R and R > 0. Aziz [1] was among the first who expanded some of the above inequalities by replacing the ordinary derivative with polar polynomial derivatives. In fact, In 1988, Aziz proved that, if P(z) is a polynomial of degree n and P(z) = 0 in |z| < k, with k ≥ 1, then for each complex number α such that |α| ≥ 1, Dividing both sides of inequality (1.7) by |α| and letting |α| → ∞, we obtain (1.3).
Over the last four decades many different authors produced a large number of results pertaining to the polar derivative of polynomials. More information on this topic can be found in the books of Milovanović et al. [11], Rahman and Schmeisser [18], and Marden [10]. One can also see in the literature (for example, refer [6,7,12,13,15,16,19]) the latest research and development in this direction. Recently, Mir [12] extended (1.7) to the class of lacunary type polynomials P(z) = a 0 + n ν=μ a ν z ν , μ ≥ 1, not vanishing in |z| < k, k ≥ 1, by first proving an L δ -norm estimate of the polar derivative D α P(z) with |α| ≥ 1 and then from the result so obtained produced the desired generalization of (1.7). More precisely, Mir proved the following result. Theorem 1.1 (cf. [12]) If P(z) = a 0 + n ν=μ a ν z ν , 1 ≤ μ ≤ n, is a lacunary polynomial of degree n having no zero in |z| < k, where k ≥ 1, then for every δ > 0 and for every complex number α such that |α| ≥ 1, which clearly represents a generalization of (1.7). In the same paper, Mir [12] further generalized inequalities (1.8) and (1.10) by proving the following result.
Theorem 1.2 (cf. [12]) If P(z) = z s (a 0 + n-s j=μ a j z j ), 1 ≤ μ ≤ ns, 0 ≤ s ≤ n -1, is a polynomial of degree n having s-fold zeros at the origin and the remaining ns zeros in |z| ≥ k, k ≥ 1, then for every complex number α such that |α| ≥ 1, which clearly is a generalization of (1.10).
The main aim of this paper is to further generalize and strengthen all the previous mentioned inequalities by establishing some general Zygmund-type integral inequalities for polynomials involving the polar derivative. The obtained results represent polar derivative analogues of some classical Bernstein-type inequalities in the uniform norm on the unit circle and include also several interesting generalizations and refinements of some L δ -norm inequalities for polynomials.

Main results
Here, we first prove the following more general inequality which includes not only (1.8) as a special case, but also derived polar derivative analogues of some classical Bernstein-type inequalities that relate the uniform norm of a polynomial to that of its derivative on the unit disk.
In the limiting case, when δ → ∞, the result is best possible for β = 0 and equality in (2.1) holds for P(z) = (z μ + k μ ) n/μ , where n is a multiple of μ and α ≥ 1. Setting μ = 1 in Theorem 2.1, we obtain the following result.

Corollary 2.2
If P(z) = n j=0 a j z j is a lacunary polynomial of degree n such that P(z) = 0 in |z| < k, where k ≥ 1, then for all complex numbers α and β, with |α| ≥ 1 and δ ≥ 1, Remark 2.3 For k = 1, we have S 1 = 1 and Corollary 2.2 reduces to a result of Mir and Wani [16] for δ ≥ 1. Dividing the two sides of (2.3) by |α| and letting |α| → ∞, we get the following result.

Corollary 2.4
If P(z) = n j=0 a j z j is a lacunary polynomial of degree n such that P(z) = 0 in |z| < k, where k ≥ 1, then for each complex number β and δ ≥ 1, 2π 0 e iθ P e iθ + nβ where S 1 and B δ are as defined in Corollary 2.2.
Taking β = 0 in Theorem 2.1, we obtain the following result.
In the limiting case, when δ → ∞, the result is best possible and equality in (2.5) holds for P(z) = (z μ + k μ ) n/μ , where n is a multiple of μ and α ≥ 1.
It is important to mention that inequalities involving polynomials in the uniform norm on the unit circle in the complex plane are special cases of the polynomial inequalities involving the integral norm. For example, if we let δ → ∞ in (2.5), noting that C δ (S 0 (μ)) → 1/(1 + S 0 (μ)), we get the following refinement of (1.10).

Corollary 2.8
If P(z) = a 0 + n j=μ a j z j , 1 ≤ μ ≤ n, is a lacunary polynomial of degree n having no zeros in |z| < k, k ≥ 1, then for each complex number α such that |α| ≥ 1, we have where S 0 (μ) is as defined in formula (1.6).
As an application of Corollary 2.5, we shall also prove the following result which provides a generalization of (1.11) and many other related results.

Theorem 2.9
If P(z) = z s (a 0 + n-s j=μ a j z j ), with 0 ≤ s ≤ n -1 and 1 ≤ μ ≤ ns, is a lacunary polynomial of degree n having s-fold zeros at the origin and the remaining ns zeros in |z| ≥ k, where k ≥ 1, then for every complex number α such that |α| ≥ 1 and δ ≥ 1, the following inequality In the limiting case, when δ → ∞, the result is best possible for s = 0 and equality in (2.7) holds for P(z) = (z μ + k μ ) n/μ , where n is a multiple of μ and α ≥ 1.
where ψ s (μ) is as defined in Theorem 2.9. It is easy to verify that, for every complex number α with |α| ≥ 1 and k ≥ 1, the function is a nonincreasing function in x. If we combine this fact with Lemma 3.5, according to which ψ s (μ) ≥ k μ for μ ≥ 1, we find that the right-hand side of (2.8) does not exceed the right-hand side of (1.12). Thus (2.8) represents a refinement of (1.12).

Auxiliary results
In order to prove our main results (Theorems 2.1 and 2.9), we need the following lemmas.
The above lemma is implicit in Qazi [17]. The next two lemmas have been recently proved by Govil and Kumar [6].

Lemma 3.2
If p and q are arbitrary positive real numbers such that q ≥ px, where x ≥ 1, and if β is any real number such that 0 ≤ β < 2π , then for each y ≥ 1.

Lemma 3.3
For real numbers a ≥ 1, b ≥ c ≥ 1, and δ > 0, we have the following inequality: We need also the following lemma proved by Aziz and Shah [2].

Lemma 3.4 Let P(z) be a polynomial of degree n and Q(z) be defined by Q(z) = z n P(1/z).
Then, for every δ > 0, the following inequality Then and suppose that φ(z) = 0 in the disc |z| < k, where k ≥ 1. Then on applying inequality (3.2) of Lemma 3.1 to the polynomial φ(z) of degree ns, we get which is equivalent to from which Lemma 3.5 follows.

Conclusion
We establish some Zygmund-type integral inequalities for the polar derivative of polynomials that are inspired by the classical Bernstein-type inequalities that relate the sup-norm of a polynomial to that of its derivative on the unit circle. The obtained results sharpen and generalize some already known estimates that relate the L δ -norms of polynomials and their polar derivatives.