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2. CARACTERIZACIÓN DE LA INSTITUCIÓN OBJETO DE ESTUDIO
The concept of quasi-interpolation (and more specifically quasi-projection, cf. e.g. [68, 89]) is vital to many approximation results in univariate and multivariate set- tings, and it is inevitably encountered alongside the concept of polynomial repro-
duction. The basic idea for both is simple, and the theoretical results achievable
thereby have important practical consequences in many approximation methods, one of which, based on tensor product splines, we have already announced to re- vise in further detail hereafter. But before we come to that, we will briefly revise the general concepts of quasi-interpolation and quasi-projection. We begin with the following definition (cf. [68]):
3.5 Definition Let F be an arbitrary Banach space of functions. Let (𝜑𝑖)𝑖∈𝐼 be a family of linearly independent elements of F that span a subspace F1 and satisfy ∣∣𝜑𝑖∣∣F ≤ 𝑏 for fixed 𝑏 and any 𝑖 ∈ 𝐼. Let further (Λ𝑖)𝑖∈𝐼 be a family of uniformly bounded functionals on F, so ∣∣Λ𝑖∣∣ < c for some global constant c > 0. Then the corresponding quasi-interpolant for any 𝑓 ∈ F is given as
Ϙ𝑓 = ∑ 𝑖∈𝐼
Λ𝑖(𝑓) ⋅ 𝜑𝑖.
If we take the special case of Lebesgue spaces with index 𝑝 ∈ ]1, ∞[ over ℝ𝑑 and choose its Hölder dual 𝑝⋆ = 𝑝/(𝑝 − 1), then a quasi-projection is the operation
Q𝑓 = ∑ 𝑖∈𝐼
⟨𝑓, 𝜑⋆𝑖⟩ ⋅ 𝜑𝑖 with ⟨𝑓, 𝜑⋆𝑖⟩ =
∫
𝑓 ⋅ 𝜑⋆𝑖, for functions {𝜑⋆𝑖}𝑖∈𝐼 ⊆ L𝑝⋆(ℝ𝑑) with ∣∣𝜑⋆𝑖∣∣L𝑝⋆
≤ 𝑏. The quasi-projection is further called a local quasi-projection if there is a fixed 𝑎0 > 0 and a set of points {ζ𝑖}𝑖∈𝐼 such that any cube of unit length in ℝ𝑑 contains at most ո0 of these points for a fixed ո0 ∈ ℕ and it holds
supp 𝜑𝑖∪ supp 𝜑⋆𝑖 ⊆ [−𝑎0, 𝑎0]𝑑+ ζ𝑖.
3.6 Remark: Fixed quasi-interpolation operators Ϙ can yield others by simple
scaling, and provided the operators are scaled suitably, the operator norms of Λ𝑖 and its ℎ-scaled version Λ𝑖,ℎcoincide. In particular (cf. [68]), fixed quasi-projection operators Q yield arbitrary ones by scaling and we obtain for 1/𝑝 + 1/𝑝⋆= 1
Qℎ𝑓 = ∑ 𝑖∈𝐼
⟨𝑓, ℎ−𝑑/𝑝⋆
𝜑⋆
𝑖(⋅/ℎ)⟩ ⋅ ℎ−𝑑/𝑝𝜑𝑖(⋅/ℎ).
The respective spaces spanned by the {𝜑𝑖(⋅/ℎ)} will consequently be denoted by Fℎ∶={𝜑𝑖(⋅/ℎ) ∶ 𝑖 ∈ 𝐼}.
Common examples of function spaces and quasi-interpolation techniques feature particularly splines (cf. [81, 89] and see below) and also for example moving least
squares (cf. [105, Ch. 4]). Both examples also come along with the second con-
cept featured in the section title, the reproduction of polynomials. In fact it is this property that makes up the key ingredient of their approximation power:
3.7 Definition A quasi-interpolation operator Ϙ is said to provide polynomial
reproduction of order 𝑚 ∈ ℕ if
Ϙ 𝑝 = 𝑝 for all 𝑝 ∈ P𝑚(ℝ𝑑).
Following this definition, a suitable result for approximation in fractional Sobolev spaces is given in the literature, to be found in [68, Sect. 3/5]. There, the result is presented in terms of integer Sobolev spaces and Besov spaces, but fractional Sobolev spaces appear as special cases of these (cf. Appendix Sect. 9.2.3ff.).
3.8 Theorem Let {Qℎ}0<ℎ<ℎ
0 be a family of local quasi-projection operators re-
producing P𝑚 on ℝ𝑑.
1. Let the basis functions {𝜑𝑖}𝑖∈𝐼 satisfy ∣∣𝜑𝑖∣∣H𝑟(ℝ𝑑)≤ 𝑏0〈1〉 and ∣∣𝜑⋆𝑖∣∣L2(ℝ𝑑)≤ 𝑏0 for
fixed 𝑏0 and some 0 < 𝑟 < 𝑚. Then we have for 0 < ℎ < ℎ0, 0 ≤ 𝜚 < 𝑟 and
𝐹 ∈ H𝑟(ℝ𝑑) that
∣∣𝐹 − Qℎ𝐹∣∣H𝜚(ℝ𝑑)≤ c ℎ
𝑟−𝜚||𝐹||
H𝑟(ℝ𝑑).
〈1〉The condition on 𝜑
𝑖 featured in [68] was actually ∣∣𝜑𝑖∣∣B𝑟
𝑝,∞
≤ 𝑏0 for Besov space B𝑟𝑝,∞, but this is
clearly implied in our setting by our condition ∣∣𝜑𝑖∣∣H𝑟(ℝ𝑑)≤ 𝑏0 due to the embedding stated in [96,
2. Let the basis functions {𝜑𝑖}𝑖∈𝐼 satisfy ∣∣𝜑𝑖∣∣H𝑚−1(ℝ𝑑)≤ 𝑏0 and ∣∣𝜑⋆𝑖∣∣L
2(ℝ𝑑)
≤ 𝑏0 for
fixed 𝑏0 > 0. Consider H𝑚(ℝ𝑑). Then we have for 0 < ℎ < ℎ
0, 𝜚 ∈ [0, 𝑚 − 1]
and 𝐹 ∈ H𝑚(ℝ𝑑) that
∣∣𝐹 − Qℎ𝐹∣∣H𝜚(ℝ𝑑)≤ c ℎ
𝑚−𝜚||𝐹||
H𝑚(ℝ𝑑).
Proof: The proof for the first case is given in [68]. The second relation is also given
in [68] for ℝ𝑑and integer orders, so 𝜚 = 𝜇 ∈ ℕ0. We use the interpolation property from Prop. 2.30 to generalise these to reals: As the relation is valid for integers, we can particularly deduce that the operator norm of Id − Qℎ as an operator from H𝑚 to H𝜇 for any integer 0 ≤ 𝜇 < 𝑚 is c ℎ𝑚−𝜇. So we have that relation for ⌊𝜚⌋ and ⌈𝜚⌉ in particular. Then we obtain with 𝜃 ∈ ]0, 1[ and 𝜚 = 𝜃⌊𝜚⌋ + (1 − 𝜃)⌈𝜚⌉ that
∣∣Id − Qℎ∣∣
H𝑚→H𝜚 ≤ c (ℎ
𝑚−⌊𝜚⌋)𝜃(ℎ𝑚−⌈𝜚⌉)(1−𝜃)= c ℎ𝑚−𝜃⌊𝜚⌋−(1−𝜃)⌈𝜚⌉ = c ℎ𝑚−𝜚. q The results of this theorem can be generalised to obtain the same convergence rates also on arbitrary Ω ∈ 𝕃𝕚𝕡𝑑 at least theoretically, and there are several op- tions to do so: The first is that for any 𝜑𝑖 such that supp 𝜑𝑖(⋅/ℎ) ∩ Ω ≠ ⌀ it holds supp 𝜑⋆𝑖(⋅/ℎ) ⊆ Ω. In this case, the results generalise by definition and we obtain in particular for admissible choices of 𝜚
∣∣𝐹 − Qℎ𝐹∣∣H𝜚(Ω)≤ c ℎ
𝑟−𝜚||𝐹|| H𝑟(Ω),
and equivalent adaptions of the second statement of the theorem. The second option is that 𝐹 is actually known in some Ω¤ ∈ 𝕃𝕚𝕡∗
𝑑such that Ω ⋐ Ω¤. In this case, we can apply the extension operator EΩ¤ and obtain 𝐹¤ = EΩ¤𝐹 that coincides with
𝐹 on Ω. By the locality property we have then for sufficiently small ℎ > 0 that whenever
supp 𝜑𝑖(⋅/ℎ) ∩ Ω ≠ ⌀ and supp 𝜑⋆𝑖(⋅/ℎ) ∩ Ω ≠ ⌀ we have also
supp 𝜑𝑖(⋅/ℎ) ⊆ Ω¤ and supp 𝜑⋆
𝑖(⋅/ℎ) ⊆ Ω¤.
In this case, we obtain with the identification of Qℎwith QℎEΩ¤(that does not affect
the result on Ω by locality) for admissible choices of 𝜚 that ∣∣𝐹 − Qℎ𝐹∣∣
H𝜚(Ω)≤ c ℎ
𝑟−𝜚∣∣𝐹¤∣∣
H𝑟(Ω¤).
Again, comparable adaptions of the second statement of the theorem can be de- duced similarly. The third option is to define Qℎdirectly by identifying Qℎ= QℎEΩ
for the universal continuous extension operator EΩ. Then we obtain, at least the- oretically, for admissible choices of 𝜚
∣∣𝐹 − Qℎ𝐹∣∣
H𝜚(Ω)≤ c ℎ
𝑟−𝜚||𝐹|| H𝑟(Ω).
As before, we can deduce equivalent adaptions of the second statement of the theorem.
3.9 Remark: These results will prove useful in a two-stage approximation method
presented in a later chapter, while we will hardly ever use them otherwise. There, we will be in a situation of a compactly supported objective function, and thus the extension to all of ℝ𝑑is easily accomplished.
3.2.2 Interpolating Quasi-Projections
As our final objective in the treatment of general quasi-projection operators, we are now going to investigate if we can enhance them such that Qℎ𝑓 interpolates 𝑓 in some finite, fixed set Ξ as long as ℎ is small enough. We will see that we can indeed do so, and this will become very important in future: Once we study approxima- tion in terms of energy functionals in a later chapter, we wish to make use of the convergence orders that quasi-projection operators provide as benchmarks. But sometimes our functionals are only given under strict interpolation constraints, for example if we want to minimise the ESM-equivalent of the energy
∫
ℝ𝑑
∑ |𝛼|=2
(𝜕𝛼𝐹)2
within the convex set of all functions in H2(Ϻ) that interpolate given function val- ues in a given finite set Ξ = {ξ1, ..., ξ𝑛} ⊆ Ϻ.
Consequently, our objective is now to determine how such an operator QΞℎ can be constructed at least theoretically to provide us with a benchmark for the ”achiev- able” approximation order. The key ingredient to this is the idea of suitably blend- ing the operator Qℎ with an interpolation operator IΞℎ in the form
QΞℎ ∶= Qℎ+ I Ξ ℎ− I
Ξ ℎ⋅ Qℎ.
This idea was for example proposed in [102], and one directly verifies or checks there that QΞℎ𝑓 will indeed interpolate a given function 𝑓 in all ξ ∈ Ξ.
What we still need now is the interpolation operator IΞℎ, and we would require it for all 0 < ℎ < ℎ0as long as ℎ0 is sufficiently small. We will now present a suitable construction method:
First of all, we restrict ourselves to local quasi-projection operators, which will be sufficient for our later treatment. Due to the C∞-version of Urysohn’s Lemma (cf. [78, Sect. 4.4]), we can find an arbitrarily smooth function 𝜑ξ ∶ Ω → [0, 1] to any ξ ∈ Ξ that is constantly 1 on B𝑑𝑞Ξ
4
(ξ) and has support in B𝑑𝑞Ξ 2
(ξ), where we define as usual the separation distance 𝑞Ξ by
𝑞Ξ∶= min ξ1,ξ2∈Ξ
∣∣ξ1− ξ2∣∣ 2.
Now we define an interpolation operator IΞ0 as
IΞ0𝑓 ∶= ∑ ξ∈Ξ
𝑓(ξ)𝜑ξ.
Since Ξ does not vary when changing ℎ, this function is fixed for any scaling factor ℎ of operator Qℎ, and moreover we have for the Sobolev Hilbert space H𝑟
‖IΞ0𝑓‖H𝑟 ≤ maxξ∈Ξ |𝑓(ξ)| ∑ ξ∈Ξ
‖𝜑ξ‖H𝑟.
Furthermore, we have as long as 𝑟 > 𝑑
2 that H 𝑟
↪ C by the Sobolev embedding theorem, and we can thereby deduce that
‖IΞ0𝑓‖H𝑟 ≤ cΞ‖𝑓‖H𝑟.
Now we have to choose ℎ0small enough such that QℎIΞ0𝑓 is indeed interpolating 𝑓 at the points of Ξ. That this is actually possible is a consequence of Q being a local quasi-projection operator by assumption and the fact that Ξ is fixed while ℎ can be chosen small: We only have to wait until any basis function and functional for Qℎ relevant for the function value at a specific ξ has its whole support contained in B𝑑𝑞Ξ
4 (ξ). Then if we choose ℎ0
so small, we obtain that QℎIΞ0𝑓 is interpolating 𝑓 at any point of Ξ for any ℎ < ℎ0, thereby defining a suitable operator IΞℎ ∶= QℎIΞ0. With this operator, we can now define QΞℎ as
QΞℎ ∶= Qℎ+ IΞℎ− IΞℎ⋅ Qℎ.
3.10 Remark: It is also worth noting that if Qℎis a projection operator, so Qℎ𝑓 = 𝑓 for 𝑓 ∈ Fℎ, then QΞℎ is as well a projection operator, as for any 𝑓 ∈ Fℎ it holds
QΞℎ𝑓 = (Qℎ+ IΞℎ− IΞℎ⋅ Qℎ)𝑓 = Qℎ𝑓 + IΞℎ𝑓 − IΞℎ𝑓 = Qℎ𝑓 = 𝑓.
see that if IΞℎis bounded in the respective space, the approximation power is indeed reproduced:
‖𝑓 − QΞℎ𝑓‖H𝑟 ≤ ‖𝑓 − Qℎ𝑓‖H𝑟 + ‖IΞℎ𝑓 − IΞℎQℎ𝑓‖H𝑟 ≤ (1 + ‖IΞℎ‖H𝑟) ⋅ ‖𝑓 − Qℎ𝑓‖H𝑟.
Taking the construction into account, then IΞℎ is bounded at least if Qℎ is bounded and if max ξ∈Ξ |𝑓(ξ)| ∑ ξ∈Ξ ‖𝜑ξ‖H𝑟 ≤ max ξ∈Ξ ‖𝜑ξ‖H 𝑟‖𝑓‖H𝑟,
with the latter being the case precisely if H𝑟 ↪ C. So we can deduce that orders are reproduced at least whenever 𝑟 > 𝑑
2 in our case, which we summarise in the following theorem:
3.11 Theorem The operator QΞℎconstructed as above from a local quasi-projection
operator Qℎ for fixed set Ξ reproduces the convergence order of Qℎ on H𝜚 when-
ever Qℎ is bounded, 𝜚 >𝑑
2 and 0 < ℎ < ℎ0 sufficiently small.
3.12 Remark: Of course, the {𝜑ξ} need not be from C∞ to make the above con- struction work, they only need to be bounded in H𝜚 at least.