#### Abstract

Uniform improving estimates of damped plane Radon transforms in Lebesgue and Lorentz spaces are studied under mild assumptions on the rotational curvature. The results generalize previously known estimates. Also, they extend sharp estimates known for convolution operators with affine arclength measures to the semitranslation-invariant case.

#### 1. Introduction

Let be a domain in and let For and , we write To avoid technical difficulties, we assume that, for and , and are finite intervals throughout the paper. For a function and a measurable function defined on , we consider the damped Radon transform defined by for . Mapping properties of such operators in various function spaces have been studied by many authors [1–9]. Sharper estimates are available in translation-invariant cases where with a function defined on an interval [10, 11] and it is widely known that the so-called affine arclength measure introduced by Drury [12] is better suited in obtaining degeneracy independent results in many interesting cases. Analogous quantity in nontranslation-invariant situation is rotational curvature, which is given by in this setting. In this paper, we are interested in uniform optimal improving properties in Lebesgue spaces and Lorentz spaces. The results will generalize known estimates for damped Radon transform and convolution operators with affine arclength measure on plane curves.

Before we state the results, we introduce certain conditions on functions defined on intervals. For an interval in , a locally integrable function , and a positive real number , we let An interesting subclass of is the collection , introduced in [13], of functions such that (1) is monotone, (2)whenever and ,

In connection with the problems related to convolution operators with affine arclength measure on curves in the plane, the author of [10] proved the following.

Theorem 1. *Let be an open interval in , and let be a function such that . Let be a nonnegative measurable function. Suppose that there exists a positive constant such that ; that is,
**
holds whenever and . Let be the operator given by
**
for . Then, there exists a constant that depends only on such that
**
holds uniformly in . *

Regarding the endpoint Lorentz space estimates, the following result due to Oberlin is available.

Theorem 2 (Oberlin [11]). *Let be a function on an interval such that on and . Then, defined in (7) maps boundedly to with the operator norm depending only on . *

In this paper, the author generalizes the aforementioned theorems to damped Radon transforms where the condition on the affine arclength measure is replaced by that on the rotational curvature. This paper is organized as follows: in Section 2, uniform estimate in Lebesgue spaces is studied, and in Section 3, endpoint Lorentz space estimate will be given based on an approach similar to Oberlin’s approach [11, 14].

#### 2. Uniform Estimates on the Plane

Theorem 3. * Let be a function on such that , and let be a nonnegative measurable function on . Suppose that there exists a positive constant such that, for each , ; that is,
**
holds whenever and . Let be the operator given by (3). Then, there exists a constant that depends only on such that
**
holds uniformly in . *

*Proof of Theorem 3. *Our proof is based on the method introduced by Drury and Guo [15], which was later refined by Oberlin [16] and the author of [10]. We have
where for and suitable functions , , defined on ,with . As in the proof of Theorem 2.1 in [10], one can show that the estimate
holds uniformly in , , , , , and . Combining this with Proposition 2.2 in the work by Christ [17] finishes the proof.

*Remark 4. *The special case in which provides a uniform estimate for the damped plane Radon transform. We write
for .

Corollary 5. *Let be a function such that . Suppose that there exists a constant such that, for each , ; that is,
**
holds whenever and . Let be the operator given by (14). Then, there exists a constant that depends only on such that
**
holds uniformly in . *

*Remark 6. *A duality argument shows the following.

Corollary 7. *Let be a function such that . Suppose that there exists a constant such that, for each , ; that is,
**
holds whenever and . Let be the operator given by (14). Then, there exists a constant that depends only on such that
**
holds uniformly in . *

#### 3. Endpoint Lorentz Estimates

Under somewhat stronger condition, estimates in Section 2 can be improved. Namely, we have the following.

Theorem 8. *Let be a function such that . Suppose that there exists a constant such that, for each , ; that is,
**
holds whenever and . Let be the operator given by (14). Then, there exists a constant that depends only on such that
**
holds uniformly in . *

*Proof of Theorem 8. *To ease our notation, we let . For a measurable subset of either or , we denote the Lebesgue measure and the characteristic function of by and , respectively.

By a well-known interpolation argument as in [2, 18], it suffices to establish the estimate
for all measurable subsets , , and of . We have
where
By Schwarz inequality, it suffices to get an estimate
uniformly in , , and . By translation invariance of in variable, it is enough to establish
uniformly in and , whereNotice that the map is one-to-one and has the absolute value of Jacobian determinant for a given .

##### 3.1. Estimate for

We follow an approach by Oberlin [14]. Letting we have Here, for , we denoted by the set . On the other hand, applying Hölder’s inequality as in [14], we get Combined with the monotonicity of , we obtain An integration in provides (25).

##### 3.2. Estimate for

For fixed and , we let where is the constant that appears in Lemma 2.2 in [11], which implies Since is nondecreasing, we see Note that the second inequality follows from a simple modification of Lemma 2.1 in [11]. This finishes the proof.

*Remark 9. *A duality argument shows the following.

Corollary 10. *Let be a function such that . Suppose that there exists a constant such that, for each , ; that is,
**
holds uniformly in . *

*Remark 11. *As is well known, if maps boundedly from to , then belongs to the convex hull of , and uniform estimates are possible only if . In the latter case, is necessary, implying the sharpness of the results. We refer interested readers to [2, 19].

#### Acknowledgment

This paper was completed with Ajou University Research Fellowship of 2011.