1 Introduction
It is challenging to construct accurate and efficient finite element schemes for solving partial differential equations in various domains. Estimations of interpolation error are important in terms of ensuring the validity of schemes and their accuracy sometimes depends on geometric conditions of meshes of the domain. Many studies have imposed the condition of shape regularity to a family of meshes
Bra07 ; BreSco08 ; Cia02 ; ErnGue04 ; Ran17 ; i.e., triangles or tetrahedra cannot be too flat in a shaperegular family of triangulations.In BabAzi76 , the shape regularity condition was relaxed to the maximumangle condition, which refers to the maximum angle of each triangle in meshes being smaller than a constant . A family of triangulations under the maximumangle condition allows the use of anisotropic finite element meshes. Anisotropic meshes have different mesh sizes in different directions, and the shape regularity assumption on triangulations is no longer valid on these meshes.
The question arises whether the maximumangle condition can be relaxed further. The answer was given by HanKorKri12 ; KobTsu14 ; KobTsu15 ; KobTsu20 ; i.e., it is known that the maximumangle condition is not necessarily needed to obtain error estimates.
The present paper proposes a general theory of interpolation error estimates for smooth functions that can be applied to, for example, Lagrange, Hermite, and Crouzeix–Raviart interpolations. For a simplex , we introduce a new geometric parameter in Section 3.7 and the error of interpolations is bounded in terms of the diameter of and . We emphasize that we do not impose the shape regularity condition and the maximumangle condition for the mesh partition.
Using the new parameter , we also propose error estimates for the Raviart–Thomas interpolation. The Raviart–Thomas interpolation error estimates on anisotropic meshes play an important role in firstorder Crouzeix–Raviart finite element analysis. In AcoDur99 , the interpolation error analysis in the lowestorder case was given under the maximumangle condition for triangles and tetrahedra. In AcoApe10 , the authors extended the results to the Raviart–Thomas interpolation with any order in two and threedimensional cases.
Meanwhile, in KobTsu18 , the lowestorder Raviart–Thomas interpolation error analysis under a condition weaker than the maximumangle condition was introduced in the twodimensional case. The analysis was based on the technique of Babuka and Aziz BabAzi76 . The technique requires a Poincarélike inequality on reference elements. However, it is not easy to deduce the inequality in the threedimensional case. To overcome this difficulty, we use the componentwise stability estimates of the Raviart–Thomas interpolation in reference elements introduced in AcoApe10 . We consequently have the Raviart–Thomas interpolation error estimates of any order in two and threedimensional cases under the relaxed mesh condition.
The remainder of the paper is organized as follows. Section 2 introduces notations and basic concepts of the Raviart–Thomas finite element. Section 3 introduces standard positions and the new geometric parameter. Further, we propose affine mappings and Piola transformations on standard positions and present the finite element generation. Section 4 proves interpolation error estimates of smooth functions that can be applied to, for example, Lagrange, Hermite, and Crouzeix–Raviart interpolations. Section 5 proves the Raviart–Thomas interpolation error estimate. Our main theorems are presented as Theorem 2 and Theorem 3.
2 Preliminaries
2.1 Function Spaces
Let . Let denote the set of nonnegative integers. Let be a multiindex. For the multiindex , let
Let be an open domain of . Let be a nonnegative integer and with . We define the Sobolev space
equipped with the norms
We use the seminorms
If , we use the notation
We set . The space is a Hilbert space equipped with the scalar product
where denotes the inner product, which leads to the norm and seminorm
The dual space of is defined and denoted by . is a Banach space with norm
For any , the norm is defined by
We introduce the function space
with the norm
Let be a matrix, and denote an operator norm as
where for .
2.2 Raviart–Thomas Finite Element on Simplices
For any , let be the space of polynomials with degree at most . is spanned by the restriction to of polynomials in , where is a closed domain. Let be a simplex. The local Raviart–Thomas polynomial space of order is defined by
(2.1) 
For
, the local degrees of freedom are given as
(2.2) 
and if ,
(2.3) 
Here,
denotes the outer unit normal vector of
on the face . For the simplicial Raviart–Thomas element in , it holds that(2.4) 
It is known that the Raviart–Thomas finite element with the set of linear forms is unisolvent; e.g., see (BofBreFor13, , Proposition 2.3.4). The triple is then a finite element.
We set the domain of the local Raviart–Thomas interpolation as ; e.g., see also (ErnGue04, , p. 27).
The local Raviart–Thomas interpolation is then defined as follows. For any ,
(2.5) 
and if ,
(2.6) 
Let with be the Raviart–Thomas finite element. Let , be an affine mapping such that with a regular matrix and . The Piola transformation is defined by
The following lemmata introduce the fundamental properties of the Piola transformation.
Lemma 1
For , , we define and . Then,
(2.7) 
Here, and are respectively the unit outward normal vectors of and .
Proof
See, for example, (BofBre08, , Lemma 3.3). ∎
By applying (2.7), we can prove the invariance of the Raviart–Thomas interpolation under the Piola transform; e.g., see (BofBre08, , Lemma 3.4).
Lemma 2
For , we have
That is to say, the diagram
commutes.
3 Standard Positions and Reference Elements
This section introduces the Jacobian matrix proposed in KobTsu20 for the threedimensional case and that proposed in KobTsu14 ; KobTsu15 ; LiuKik18 for the twodimensional case.
Let us first define a diagonal matrix as
(3.1) 
3.1 Twodimensional case
Let be the reference triangle with vertices , , and .
Let be the family of triangles
with vertices , , and .
We next define the regular matrices by
(3.2) 
with parameters
For , let be the family of triangles
with vertices . We then have , .
3.2 Threedimensional cases
There are two threedimensional cases
Let and be reference tetrahedrons with the following vertices.
 (i)

has the vertices , , .,
 (ii)

has the vertices , , ..
Let , , be the family of triangles
with vertices
 (i)

, , , and ,
 (ii)

, , , and .
We next define the regular matrices by
(3.3) 
with parameters
For , , let , be the family of triangles
with vertices
We then have , , and
3.3 Standard Positions
In what follows, we impose conditions for in the twodimensional case and in the threedimensional case.
Condition 1 (Case that )
Let with vertices () introduced in Section 3.1. We assume that is the longest edge of ; i.e., . Recall that and . We then assume that . Note that .
Condition 2 (Case that )
Let with vertices () introduced in Section 3.2. Let () be edges of . We denote by the edge of with minimum length; i.e., . Among the four edges that share an end point with , we take the longest edge . Let and be end points of the edge . We thus have
Consider cutting with the plane that contains the midpoint of the edge and is perpendicular to the vector . We then have two cases:
 (i)

and belong to the same halfspace;
 (ii)

and belong to different halfspaces.
In each case, we respectively set
 (i)

and as the end points of , that is ;
 (ii)

and as the end points of , that is .
Finally, recall that . Note that we implicitly assume that and belong to the same half space. Also note that and , where denotes the diameter of .
3.4 Affine Mappings and Piola Transforms
The present paper adopts the following affine mappings and Piola transformations.
Definition 1
Let satisfy Condition 1 or Condition 2. Let , and be the simplices defined in Sections 3.1 and 3.2. That is to say,
We define the affine mapping by
(3.4) 
Let and be the Piola transformations with respect to and , respectively. We define by , which is the Piola transformation with respect to .
3.5 Finite Element Generation on Standard Positions
We follow the procedure described in (ErnGue04, , Section 1.4.1 and 1.2.1).
For the reference element defined in Sections 3.1 and 3.2, let be a fixed reference finite element, where is a vector space of functions for some positive integer (tipically or ) and is a set of linear forms such that
is bijective; i.e., is a basis for . Further, we denote by in the local (valued) shape functions such that
Let be a normed vector space of functions such that and the linear forms can be extended to . The local interpolation operator is then defined by
(3.5) 
Let , , and be the affine mappings defined in (3.4). For , we first define a Banach space of valued functions that is the counterpart of and define a linear bijection mapping by
with two linear bijection mappings:
Furthermore, the triple is defined by
while the triple is defined by
and are then finite elements. The local shape functions are and , , and the associated local interpolation operators are respectively defined by
(3.6)  
(3.7) 
Proposition 1
The diagrams
commute.
Proof
See (ErnGue04, , Proposition 1.62). ∎
Example:
Let be a finite element.

For the Lagrange finite element of degree , we set .

For the Hermite finite element, we set .

For the Crouzeix–Raviart finite element with , we set .
3.6 Raviart–Thomas Finite Element on Standard Positions
3.7 Parameter and Mesh
We first propose a new parameter .
Definition 2
In the sequel of this paper, the interpolation errors are bounded in terms of and . However, the parameters and proposed below might be more convenient for the practical computation of finite element methods.
We assume that is a bounded polyhedral domain. Let be a simplicial mesh of , made up of closed simplices, such as
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