1. Introduction
Our experience with the application of means to several problems comes from the study of the numerical solution of conservation laws. In particular, in [
1], a new limiter based on the
mean was presented, giving rise to very efficient new methods of solving these kind of equations. This mean has the following expression:
To our knowledge, this was the first time in the literature regarding the numerical solution of conservation laws that a new limiter was presented satisfying the following criteria:
If , and , then
This property is crucial to preserve the order of approximation of the methods in areas of regularity;
In applications, this property implies adaption in the case of discontinuities.
The case
of these means coincides with the harmonic mean. These means were also used successfully in nonlinear subdivision and multiresolution schemes [
2,
3,
4]. In this field of application, it is also quite important for the mean to satisfy the Lipchitz condition; in fact, the harmonic mean satisfies this prerequisite, as do the
means, but the Lipchitz constant
p is unfortunately larger than desirable to make theory and practice workable for higher-order methods.
In [
5], the authors present a rigorous study of some specific typse of subdivision schemes, depending on a general mean
. They propose the generalized means in the article as a particular example
For these means, the required properties listed above are satisfied, and the Lipchitz constant is in these cases for This constant tends to 1 when k increases, and this is enough to allow for improvements in the theory and applications of nonlinear subdivision and multiresolution schemes, as well as in the numerical solution of conservation laws.
These means attain only two orders of approximation to the arithmetic mean in smooth areas. However, of course, one could immediately consider using the ideas introduced in [
1] to generalize them to have
p orders of approximation. Intuitively, the right candidate would be
Unfortunately, this is not the case, and the means attain only two orders of approximation to the arithmetic mean in the sense described above. This fact can be taken from the observation for whichever
From this point on, one can try slight modifications of the given expression to find useful means that satisfy the mentioned properties with appropriate Lipchitz constants and allow high orders of approximation to be maintained. To our knowledge, there is no family of means with adaptivity constants tending to with Lipchitz constant that also approach and satisfy the requirement of proximity to the arithmetic mean. Of course, one needs to be aware of proving each one of the properties carefully to ensure some basic properties are not lost while proving new ones. It is in this process that we have found an adequate expression, which opens up a variety of potential applications.
This work is organized as follows: in
Section 2, we introduce the new mean and verify the most basic properties; in
Section 3, we give the main results of the paper regarding the boundedness of the mean by a constant times the minimum of the two values, the approximation order to the arithmetic mean, and the Lipchitz condition; and finally, in
Section 5, we show in some detail why this new mean is important for several applications, and we give some conclusions and perspectives for future work.
2. The Generalized Power Means
In this section, we introduce the proposed new family of means and prove some basic properties. Let us suppose
and
; we define
for a natural number
k and a real number
in the following way:
where
is a polynomial of degree
defined by
Lemma 1. Let us consider the function for and , and let us suppose that ; then,
Proof. Let us start by proving the first point.
For the second point, it is apparent that
and for the other inequality, we compute
Now, taking into account the fact that
since
The third point requires more computation. We start with the expression of the partial derivative with respect to
x by applying simple derivation rules:
where
q stands for the denominator in the quotient of polynomials
In view of Equation (
6), we need to compute
and
:
Now, we expand each product of polynomials in Equation (
6); that is,
After extensive computations and applying the symmetry properties and basic results of combinatorial numbers, one can find that all coefficients are identically zero except the coefficient for Thus, finally,
The proof for is carried out following the same arguments. □
In the next result, we prove that is in fact a mean of the two values x and y.
Theorem 1. Let us consider and ; then, Proof. Let us suppose that
We start by proving the first inequality
by using only the first point of Lemma 1.
Now, in order to prove the inequality
we observe that since
, this implies
Thus
□
The relevance of these means is presented in the next section, in which we prove three main properties that have serious implications for the theoretical analysis of some numerical methods and also for their practical performance. They allow the definition of nonlinear methods by slightly modifying existing linear methods, generating adaptive counterparts that maintain the order of approximation (thanks to Proposition 2 in the next section), minimize the effect of discontinuities (as a consequence of Proposition 1) and retain the stability of the schemes (due to the possibility of obtaining contractive operators, due to the bound tending to 1 with the parameter
k obtained in Proposition 3). For an explicit simple example, see
Section 5.
3. Important Practical Properties of Generalized Power Means
In this section, we prove some specific properties that are essential for some applications, such as approximation theory, nonlinear subdivision and multiresolution schemes and the numerical solution of hyperbolic conservation laws. In particular, we consider the following: in the first result, we prove that the mean is bounded by a constant times the minimum argument; in the second result, we study the second-order approximation to the arithmetic mean; and in the third, we show that the mean satisfies a Lipchitz condition with an adequate constant.
Proposition 1. Let us consider and ; then, Proof. Let us suppose that Using the first point of Lemma 1, we can easily rewrite the expression of the proposed mean as and therefore, since according to the second point of Lemma 1 we have we obtain the result. □
Proposition 2. Let us consider , If , and ; then,
Proof. The proof begins with the following assertion:
Now, considering the function
defining
and applying the basic Lagrange theorem
which completes the proof. □
Proposition 3. Let us consider , and ; then, where with
Proof. Applying the mean value theorem, we obtain
where
From the expression
, computing
and
, we obtain
with
and
Plugging into
the computations of
and
conducted in points 3 and 4 of Lemma 1, we obtain
and this completes the proof with
□
4. Example of Application
The new means are well adapted to applications in some fields, such as nonlinear reconstruction operators, subdivision and multiresolution schemes. Our example is focused on a particular application to nonlinear subdivision schemes, although many other applications could be considered by simply improving some theoretical results that appear in articles that involve nonlinear means, such as the harmonic mean [
1,
4,
6]. The reason for this lies in Propositions 1–3. Proposition 3, for instance, allows us to prove the stability of the scheme that we define in the following lines.
Given a set of nested grids in
:
where
is some fixed integer, we consider the point-values
of a function
at the coarsest grid
Then, we refine the given data to finer scales according to the following definition of a subdivision scheme. First, let us introduce the second-order divided differences
that, since we are working in uniform grids, relate to the second-order finite differences
that appear at the numerator. Given this definition, and the data at a scale
s over the grid
we consider one step of the subdivision scheme given by
where
stands for any mean of the values
x and
Let us extend the definition of our mean
so that it can be used with both negative and positive values:
For the sake of simplicity, is referred to as the new extension below. We now consider the three following cases:
which gives the well known scheme of Deslauriers and Dubuc (DD) [
7];
which defines the PPH subdivision scheme [
3];
given in Equation (
4) with
We have chosen
in the expression of the mean
in order to maintain the fourth-order approximation of the scheme in areas where the data come from smooth functions. In fact, if we take into account that the original scheme of Deslauriers and Dubuc [
7] is known to be fourth-order accurate, which results from its definition using fourth-order piecewise centered Lagrange interpolation, we can also prove that the scheme based on
is fourth-order accurate. The key issue for this fact to be true is the use of Proposition 2 in order to see that if
stands for the DD subdivision at the intermediate points with odd indexes,
for the PPH subdivision and
for the generalized power means (GP) subdivision, we have
We have chosen the values at the boundaries and by interpolating the first and last four data points using basic Lagrange interpolation. The nonlinear modifications then take place in the center of the intervals.
Remark 1. In the expression for in Equation (12), the dependence on the grid space is fictitious, as the expression can be rewritten by making use of the second-order finite differences as follows:which represents the way in which it is implemented in the computer programs. It is important to notice that the second-order divided differences are indicators of the smoothness of the underlying function; in fact, the largest divided difference indicates the potential presence of a singularity in the data. Due to the boundedness of both the harmonic mean and the
means by a constant multiplied by the minimum of their two arguments (see Proposition 1), in the presence of a jump discontinuity in the data, the influence of a discontinuity in Equation (
12) is limited. This behavior will occur for any nonlinear mean with this property. Let us suppose, for example, that a jump discontinuity lies in
; then,
However,
and the contribution of the incorrect data
to Equation (
12) is neglected. Notice that, in this case, the order of approximation suffers a reduction to the second order, but it is not completely lost as in the case of the DD scheme, for example. With regard to practical properties in numerical examples, one expects similar behavior to existing nonlinear schemes based on the harmonic mean or some gain in adaption due to Proposition 1, which gives a smaller constant than the p-power means (generalizations of the harmonic mean) defined in [
1], for example, for which we obtain
p instead of
, which follows from Proposition 1 (see also the numerical experiments in
Section 4 to observe the behavior of the schemes around the discontinuities).
Following the same idea presented in [
3,
6], it is possible to prove basic properties of the presented scheme for the mean
, such as the existence of a nonlinear scheme
for the first-order differences, the convergence of the subdivision scheme, the smoothness of the limit function of the subdivision scheme, the approximation order of the schemes, the behavior around jump discontinuities, the elimination of Gibbs effects and some other interesting properties. We focus on proving the stability of the subdivision scheme, which also shows the importance of Proposition 3. We first give the following simple lemma and a proposition regarding the contractivity of the finite difference operator.
Lemma 2. The function defined in by satisfies
Proof. Since
due to Proposition 1, we find that
and
We also find that
according to the extended definition of
given in (
13). The combination of both arguments easily proves points 1 and 2.
In order to prove point 3, we use the triangular inequality and Proposition 3 as follows,
□
Proposition 4. When removing s for simplicity, where stands for the subdivision scheme with the mean ; then,
Proof. We first prove point 1; that is, for all indices
We treat odd and even indices separately.
From (
12) and (
13), we find that
and thus, using Proposition 1
- (b)
Again, from (
12) and (
13), we find that
Thus, using Lemma 2, we obtain
which proves point 1.
In order to prove point 2, we again separate odd and even indices.
Using algebraic manipulations of the last point, we find that
and now, using Proposition 3, we obtain
- (b)
In this case, we can write
by using point 3 of Lemma 2.
□
Remark 2. If in Proposition 4, then the operator Δ is said to be contractive, and this fact is a crucial issue in the proof of the next theorem. Moreover, the smaller the contractivity constant, the better. Therefore, an increase in k gives a better constant. Thus, it could be proposed that the minimum of the two arguments should be used as the expression for the mean in (12), and this is optimal with regards to adaption, but at the cost of losing the approximation order given by Proposition 2, which would be no longer true. With the mean we achieve both goals. Let us now prove a theorem that ensures the stability of the subdivision scheme.
Theorem 2. Given two initial sequences let us consider steps of subdivision where stands for the subdivision scheme (12) with the mean with ; then, with
Proof. Since
we can write
From (
14), by using Proposition 3, we obtain
Now, applying Proposition 4, we obtain
with
Applying the same reduction as in (
15) recursively, we reach
Taking into account the fact that
and
we finally obtain
with
□
Remark 3. In [
6]
, the authors prove a similar theorem for the PPH subdivision scheme following similar arguments, and a stability constant is obtained, which is larger than the value of obtained from Theorem 2 for or for In the limit, when k grows to infinity, we obtain The practical importance of having a smaller stability constant lies in the fact that, in order to ensure a prescribed tolerance ϵ in the final error, one can use approximated input data with error. Therefore, there is more flexibility to ensure the required precision. Numerical Test
Let us consider the function
defined in the interval
and given by
We consider the set of initial data given by the point value discretization of the function
in 20 equally spaced points; that is,
Our purpose is to see the result of applying linear and nonlinear subdivision schemes in the presence of a jump discontinuity and also to compare the approximation of the limit function with perturbed data
and estimate the stability constant given in Theorem 2 simply by computing,
after
levels of subdivision.
We have implemented random perturbations of the original data with the maximum absolute values
and
Then, we have computed the stability constant indicated in (
17) for the three subdivision schemes considered; that is,
and
, respectively. The obtained estimations are given in
Table 1. It can be seen that, in practice, the three methods behave in a very stable way, and that the stability constants are normally much smaller than the theoretical estimates. However, we want to point out that the bound given in Theorem 2 is sharper than the one obtained for the PPH method. We also see that the larger the
k, the better the estimation and the adaption to the expected reproduction of the original function around the jump discontinuity. It is also interesting to observe how the nonlinear subdivision schemes define the area around the discontinuity better, while the linear counterpart generates undesirable effects in this zone; see
Figure 1.
Remark 4. The means may require arithmetic with variable precision for larger values of k, and the computational time also grows considerably with k, which needs to be taken into account. Therefore, a balance between these points must be considered depending on the application. The theoretical simplicity of the proofs using this in the schemes is undeniable, which in some cases may allow for theoretical results that are much more difficult or even not possible to obtain for other means—see, for example, the work presented in [
4]
, where the authors do not reach any theoretical results regarding stability due to the difficulty of obtaining a contraction property for the finite differences, as achieved in Proposition 4 for the mean