# A New Proof of Inequality for Continuous Linear Functionals

- Feng Cui
^{1}Email author and - Shijun Yang
^{1}

**2011**:179695

https://doi.org/10.1155/2011/179695

© F. Cui and S. Yang. 2011

**Received: **23 January 2011

**Accepted: **2 March 2011

**Published: **14 March 2011

## Abstract

Gavrea and Ivan (2010) obtained an inequality for a continuous linear functional which annihilates all polynomials of degree at most for some positive integer . In this paper, a new functional proof by Riesz representation theorem is provided. Related results and further applications of the inequality are also brought together.

## 1. Introduction

and denote by the truncated power function. The notation means that the functional is applied to considered as a function of . The main result of [1] can now be stated as follows.

Theorem 1.1.

where is an arbitrary constant and the symbol denotes a th antiderivative of .

Remark 1.2.

It should be pointed out that the inequality (1.3) can be found in Wang and Han [2, Lemma 1] (see also [3]). In this note, we will give a short account of historical background on inequality (1.3). A new functional proof based on the Riesz representation theorem [4, 5] is also given. Furthermore, some related results are brought together, and further applications are also included.

## 2. Historical Background

Using (2.1), Smale was able to compute the average error for right rectangle rule, the trapezoidal rule, and Simpson's rule (see [6, Theorem D]).

Later on, Wang and Han in [2] extended and unified results in [6, Theorem D], and they also simplified the corresponding analysis given in [6]. The main observations in [2] are

(i)any quadrature rule has its algebraic precision, or equivalently, the corresponding quadrature error functional annihilates some polynomials,

(ii)and hence the Peano kernel theorem applies.

Then, is a Hilbert space of functions. The result in [2] can be now stated as follows.

Theorem 2.1.

It is easily seen that Theorem 1.1 is a rediscovery of Theorem 2.1. For completeness, we record the original short but beautiful proof of (2.4) in [2].

Proof.

Applying the functional to both sides of (2.10) and noting (2.7) and (2.8), we obtain (2.4) as required.

## 3. A Functional Proof

It seems that the original proof of Wang and Han recorded in the previous section does not fully utilize the space . We now provide an alternative functional proof.

*~*on with respect to its subspace since vanishes on . We say that if . It is easy to check that the quotient space is still a Hilbert space. For any , there must exist a function such that , . For example,

as desired.

Remark 3.1.

From the above proof, we see that given by (3.8) is the representer of the Hilbert space .

## 4. Related Results and Further Applications

Numerical integration and quadrature rules are classical topics in numerical analysis while quadrature error functionals are typical continuous linear functionals on function spaces. It was quadrature error functionals that stimulated study of Smale [6] and Wang and Han [2]. So it is natural to consider the applications of (2.4) to quadrature error estimates.

Example 4.1.

Example 4.2.

where is the Bernoulli function, defined by . Here stands for the fractional part of .

Example 4.3.

Note that there is a mistake in Example 9 in [1]. The constant in the last inequality is mistaken to be there.

Recently, there is a flurry of interest in the so-called Ostrowski-Grüss-type inequalities. Some authors, for example, see Ujević [9], consider to bound a quadrature error functional in terms of the Chebyshev functional, that is, , for some appropriate integer (see, e.g., [9]). It is worth mentioning that these Ostrowski-Grüss-type inequalities are related to inequality (1.3). Actually, we have the following general result.

Proposition 4.4.

Proof.

From (4.12)–(4.14) and (2.4), follows (4.9). This completes the proof.

Note that Proposition 4.4 shows that we have a corresponding inequality (4.9) for every whenever we have inequality (2.4). It should be mentioned, however, (4.9) does not hold for in general especially when the kernel of is exactly .

Proposition 4.4 can be reformulated in a slightly different language as follows.

Corollary 4.5.

Suppose that is a continuous linear functional acting on and . Then for any nonnegative , both (2.4) and (4.9) hold while only (2.4) is also valid for .

Finally, we end this paper with an inequality of the above-mentioned Grüss-type. More examples are left to the interested readers.

Example 4.6 (see also [7]).

In view of Proposition 4.4 or Corollary 4.5, the above inequality is still valid with replaced by and , respectively, and with obvious change in the coefficients. We omit the details.

## Declarations

### Acknowledgment

The work is supported by Zhejiang Provincial Natural Science Foundation of China (Y6100126).

## Authors’ Affiliations

## References

- Gavrea I, Ivan M:
**An inequality for continuous linear functionals.***Applied Mathematics Letters*2010,**23**(4):381–384. 10.1016/j.aml.2009.11.002MathSciNetView ArticleMATHGoogle Scholar - Wang XH, Han DF:
**Computational complexity on numerical integrals.***Science in China. Series A*1991,**34**(1):42–47.MathSciNetMATHGoogle Scholar - Wang XH:
**Some results relevant to Smale's reports.**In*From Topology to Computation: Proceedings of the Smalefest (Berkeley, CA, 1990)*. Edited by: Hirsch MW, Marsden JE, Shub M. Springer, New York, NY, USA; 1993:456–465.View ArticleGoogle Scholar - Conway JB:
*A course in Functional Analysis, Graduate Texts in Mathematics*.*Volume 96*. 2nd edition. Springer, New York, NY, USA; 1990:xvi+399.Google Scholar - Bollobás B:
*Linear Analysis: An Introductory Course*. 2nd edition. Cambridge University Press, Cambridge, UK; 1999:xii+240.View ArticleMATHGoogle Scholar - Smale S:
**On the efficiency of algorithms of analysis.***Bulletin of the American Mathematical Society*1985,**13**(2):87–121. 10.1090/S0273-0979-1985-15391-1MathSciNetView ArticleMATHGoogle Scholar - Ren H, Yang S:
**A general inequality of Grüss type.***Journal of Inequalities in Pure and Applied Mathematics*2006,**7**(2, article 54):1–6.MathSciNetGoogle Scholar - Yang S:
**A unified approach to some inequalities of Ostrowski-Grüss type.***Computers & Mathematics with Applications*2006,**51**(6–7):1047–1056. 10.1016/j.camwa.2005.09.005MathSciNetView ArticleMATHGoogle Scholar - Ujević N:
**Sharp inequalities of Simpson type and Ostrowski type.***Computers & Mathematics with Applications*2004,**48**(1–2):145–151. 10.1016/j.camwa.2003.09.026MathSciNetView ArticleMATHGoogle Scholar

## Copyright

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.