Some new sharp limit Hardy-type inequalities via convexity
© Barza et al.; licensee Springer. 2014
Received: 29 August 2013
Accepted: 3 December 2013
Published: 2 January 2014
Some new limit cases of Hardy-type inequalities are proved, discussed and compared. In particular, some new refinements of Bennett’s inequalities are proved. Each of these refined inequalities contain two constants, and both constants are in fact sharp. The technique in the proofs is new and based on some convexity arguments of independent interest.
holds for all measurable and non-negative functions f on , whenever , .
The constant in (1.2) is sharp. There exists nowadays a lot of information about Hardy-type inequalities, see, e.g., the books [4, 5] and the references given there. In particular, in these books it is pointed out that such inequalities are specially important for a great variety of applications, e.g., to the theory of function spaces, interpolation theory, approximation theory, partial differential equations, etc.
However, there exist very few Hardy-type inequalities with sharp constant in the limit case and when the interval is replaced by a finite interval , . We continue by giving two such examples, where the first one (Bennett’s inequalities) also has direct applications, e.g., to interpolation theory (see Remark 1.1 below).
with the usual modification if .
Proof The proof is given in  for . For completeness we present a short proof for . We consider first (1.3).
Remark 1.1 This result is due to Bennett . He derived it as an important tool when he described the intermediate spaces between L and with the Peetre real method. This result was later on completed and applied in various ways in, e.g., [7–9]. In fact, the constant in both (1.3) and (1.4) is sharp. This was not pointed out in , but it is a consequence of our Theorem 2.3.
However, this inequality is not best possible, but also a ‘sharp’ variant of this inequality is known.
The constant is sharp.
(which holds by Jensen’s inequality) and also to a number of other Hardy-type inequalities (see Theorem 1.3 in ).
In Section 2 of this paper, we prove a refined version of Proposition A, where all involved constants are sharp (see Theorem 2.3 which, in particular, shows that the constant in Proposition A is sharp in both (1.1) and (1.2)). Up to our knowledge, there is not known any other Hardy-type inequalities with this property. The method of proof is completely different from that in  and is based on a convexity argument (see Lemma 2.1 and Remark 2.2).
In Section 3 we present some further results and remarks. In particular, we use the idea from the proof of Theorem 2.1 in  to derive a sharp inequality of the same type as those in Proposition A (see Proposition 3.3 and Example 3.6) and compare these results. Also in this case the proof is based on a convexity argument. Moreover, we discuss the connections between the results above (e.g., that Proposition A is in a sense almost a limit case of Proposition B when and ). All inequalities we derive are sharp but the optimal test functions are different. Therefore, we can in particular formulate another strict improvement of Proposition A (see, e.g., Remark 3.5 and Example 3.6).
2 A refinement of Proposition A
The following well-known lemma of independent interest will be used in the proof of Theorem 2.3.
for all . Equality holds if and only if .
Remark 2.2 The crucial inequality (2.1) is called ‘a fundamental relationship’ in the book  (p.12). Two proofs are pointed out in this book. Another proof follows by observing that is convex for or and concave for , and that the equation of the tangent at the point is . In the general case, the tangent will be , which implies a generalization of (2.1), when is convex/concave.
The main result in this section is a refinement and extension of Proposition A.
- (a)If , then(2.2)and(2.3)
Both constants and in (2.2) and (2.3) are sharp. Equality is never attained unless f is identically zero.
If , then both (2.2) and (2.3) hold in the reverse direction and the constants in both inequalities are sharp. Equality is never attained unless f is identically zero.
If we have equality in (2.2) and (2.3) for any measurable function f and any .
Hence, we have proved that (2.2) holds for all continuous functions. By standard approximation arguments, (2.2) holds for all measurable functions.
Hence, by letting , we obtain that . This shows that is the sharp constant in (2.4) and consequently in (2.2).
and argue similarly as before. The proof of the sharpness of (2.3) consists only of small modifications of the proof above. By Lemma 2.1 it is clear that we cannot have equality neither in (2.2) nor in (2.3) unless f is identically zero. The proof is complete.
(b) Let . In this case the crucial convexity inequality from Lemma 2.1 holds in the reversed direction. Hence, the proofs of the reverse of (2.2) and (2.3) consist only of small modifications of the proofs of (2.2) and (2.3), respectively.
(c) The equality for in both (2.2) and (2.3) is just a consequence of partial integration and limiting arguments or of straightforward modifications of the proof above. □
Remark 2.4 Easy calculations show that if we get equality in inequality (1.5) for . Hence, in this case, inequality (1.5) cannot be improved in the same manner as above to a refined inequality of the type (2.2). In the same way, we find that for the case inequality (1.6) cannot be improved to some refined inequality of the type (2.3).
3 Further results and remarks
Remark 3.1 We note that, by making the variable transformation , the result in Proposition A can be formulated for the interval instead of . Hence, it can be compared with that of Proposition B. The same argument shows that it is no loss of generality to formulate Proposition B with only for , so in the sequel we consider only this case.
holds and that the constant is sharp.
Inspired by Remark 3.2 and the technique used in  to prove (1.7), we formulate the following estimate of the same type as that in Proposition A.
(for the case , we assume that ).
If , then (3.3) holds in the reverse direction. Inequality (3.3) and the reverse inequality for are sharp and equality holds for , .
Since for we get equality in our inequality, the sharpness statement is also proved. The proof for the case follows similarly, since the only inequality above holds in the reverse direction. □
where . Both inequalities (1.3) and (3.4) are sharp but the optimal test functions are completely different so the two results cannot be compared.
Moreover, a similar improvement of (1.4) can be stated.
as a limit case of the Hardy inequality (corresponding to the case ).
Remark 3.7 The simple proof of Proposition 3.3 shows that it can be generalized also to multidimensional cases and to integral inequalities with general measures. We can also formulate Proposition 3.3 in terms of a general convex function instead of the special case , cf.  and the references given there.
Remark 3.8 By using suitable variable transformations, all inequalities in this paper can be reformulated over the interval instead of over the interval .
Finally, we present the following variant of Theorem 2.3.
- (a)If , then(3.5)and(3.6)
Both constants and in (3.5) and (3.6) are sharp. Equality is never attained unless f is identically zero.
If , then both (3.5) and (3.6) hold in the reverse direction and the constants in both inequalities are sharp. Equality is never attained unless f is identically zero.
If , we have equality in (3.5) and (3.6) for any measurable function f and any .
Proof Substitute by in Theorem 2.3 and make a change of variables. □
The work of the third named author, Natasha Samko, was supported by FCT-CEAF (Portugal) under the project PEst-OE/MAT/UI4032/2011. The first named author would also like to thank the group of Applied Modeling of Karlstad University for support. We thank both referees for some good suggestions, which helped to improve the final version of this paper.
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