# Linear isoparametrics with dual finite elements

The subject presented here is some content of the Wikipedia page about Platonic solids combined with my own experience on Finite Elements.
To start with the latter, there is a standard piece of Finite Element theory concerning triangles on MSE. The concept of isoparametrics is introduced herein. A reference to the same theory is found in: Is there any equation for triangle? (MSE).
So far so good for 2-D. In three dimensions the analogue of a triangle is a tetrahedron. Let the parent tetrahedron have vertices (Finite Element "nodes") that are numbered as follows: $$(0) = (0,0,0) \quad ; \quad (1) = (1,0,0) \quad ; \quad (2) = (0,1,0) \quad ; \quad (3) = (0,0,1)$$ Then any function with the parent tetrahedron can be interpolated as: $$f(\xi,\eta,\zeta) = (1-\xi-\eta-\zeta)f_0 + \xi\,f_1 + \eta\,f_2 + \zeta\,f_3$$ Isoparametrics means that the same interpolation holds for the (global) coordinates as well: $$x = (1-\xi-\eta-\zeta)x_0 + \xi \, x_1 + \eta \, x_2 + \zeta \, x_3 \\ y = (1-\xi-\eta-\zeta)y_0 + \xi \, y_1 + \eta \, y_2 + \zeta \, y_3 \\ z = (1-\xi-\eta-\zeta)z_0 + \xi \, z_1 + \eta \, z_2 + \zeta \, z_3$$ From this the local parameters $(\xi,\eta,\zeta)$ can eventually be solved and expressed in the global coordinates (which is not common practice, though).

Another frequently used Finite Element is the quadrilateral in 2-D or, quite analogously in 3-D: the hexahedron. The nodes of our parent element are: $$(1) = (-1,-1,-1) \quad ; \quad (2) = (+1,-1,-1) \quad ; \quad (3) = (-1,+1,-1) \quad ; \quad (4) = (+1,+1,-1) \quad ; \quad (5) = (-1,-1,+1) \quad ; \quad (6) = (+1,-1,+1) \quad ; \quad (7) = (-1,+1,+1) \quad ; \quad (8) = (+1,+1,+1)$$ Then any function $h$ at the parent $h$exahedron can be interpolated as: $$h(\xi,\eta,\zeta) = \frac{1}{8}(1-\xi)(1-\eta)(1-\zeta) h_1 + \frac{1}{8}(1+\xi)(1-\eta)(1-\zeta) h_2 + \frac{1}{8}(1-\xi)(1+\eta)(1-\zeta) h_3 + \frac{1}{8}(1+\xi)(1+\eta)(1-\zeta) h_4 + \frac{1}{8}(1-\xi)(1-\eta)(1+\zeta) h_5 + \frac{1}{8}(1+\xi)(1-\eta)(1+\zeta) h_6 + \frac{1}{8}(1-\xi)(1+\eta)(1+\zeta) h_7 + \frac{1}{8}(1+\xi)(1+\eta)(1+\zeta) h_8$$ Note that this interpolation, in general, is not linear. And it's not so easy anymore to solve for the global coordinates ( by substituting $h = x,y,z$ ).

The following is a possible generalization of a finite element which was questioned about in: Understand 1D FEM solution using quadratics elements (MSE). In three dimensions we have the quite common Finite Difference Star, which is a not so common Finite Element, namely an octahedron augmented with the origin as an additional node $(0)$: $$(0) = (0,0,0) \quad ; \quad (1) = (-1,0,0) \quad ; \quad (2) = (+1,0,0) \quad ; \quad (3) = (0,-1,0) \quad ; \quad (4) = (0,+1,0) \quad ; \quad (5) = (0,0,-1) \quad ; \quad (6) = (0,0,+1)$$ Then any function $s$ with the parent octahedron (F.D. $s$tar) can be interpolated as: $$s(\xi,\eta,\zeta) = (1 - \xi^2 - \eta^2 - \zeta^2) s_0 + \frac{1}{2}\xi (\xi-1) s_1 + \frac{1}{2}\xi (\xi+1) s_2 + \frac{1}{2}\eta (\eta-1) s_3 + \frac{1}{2}\eta (\eta+1) s_4 + \frac{1}{2}\zeta (\zeta-1) s_5 + \frac{1}{2}\zeta (\zeta+1) s_6$$ But now Wikipedia is consulted and the following is read in the chapter Dual polyhedra:

• The tetrahedron is self-dual (i.e. its dual is another tetrahedron).
• The cube and the octahedron form a dual pair.
Therefore let's construct first the dual of an arbitrary octahedron, which is not quite an arbitrary hexahedron: $$h_1 = (s_1 + s_3 + s_5)/3 \quad ; \quad h_2 = (s_2 + s_3 + s_5)/3 \quad ; \quad h_3 = (s_1 + s_4 + s_5)/3 \quad ; \quad h_4 = (s_2 + s_4 + s_5)/3 \quad ; \quad h_5 = (s_1 + s_3 + s_6)/3 \quad ; \quad h_6 = (s_2 + s_3 + s_6)/3 \quad ; \quad h_7 = (s_1 + s_4 + s_6)/3 \quad ; \quad h_8 = (s_2 + s_4 + s_6)/3$$ Substitute this into the abovementioned formula for $h(\xi,\eta,\zeta)$ and then simplify. The outcome (with a little help from Maple) is, to my great surprise: $$\frac{1}{6}\left[\, (s_1 + s_2 + s_3 + s_4 + s_5 + s_6) + (s_2 - s_1)\,\xi + (s_4 - s_3)\,\eta + (s_6 - s_5)\,\zeta\, \right]$$ The interpolation with the dual hexahedron of an arbitrary octahedron is linear. Because of the ispoarametrics ( substitute $h=x,y,z$ ), it also means that the coordinates of the dual are far from arbitrary - but we did already know that, didn't we?

The other way around, we construct the dual of an arbitrary hexahedron, which is (not quite) an arbitrary octahedron: $$s_1 = (h_1 + h_3 + h_5 + h_7)/4 \quad ; \quad s_2 = (h_2 + h_4 + h_6 + h_8)/4 \\ s_3 = (h_1 + h_2 + h_5 + h_6)/4 \quad ; \quad s_4 = (h_3 + h_4 + h_7 + h_8)/4 \\ s_5 = (h_1 + h_2 + h_3 + h_4)/4 \quad ; \quad s_6 = (h_5 + h_6 + h_7 + h_8)/4 \\ s_0 = (s_1 + s_2 + s_3 + s_4 + s_5 + s_6)/6$$ Substitute this into the abovementioned expression for $s(\xi,\eta,\zeta)$ and simplify. The outcome (with a little help from Maple) is, again to my surprise: $$\frac{1}{8}\left[ \,( h_1+h_2+h_3+h_4+h_5+h_6+h_7+h_8) \\ + (-h_1+h_2-h_3+h_4-h_5+h_6-h_7+h_8)\xi \\ + (-h_1-h_2+h_3+h_4-h_5-h_6+h_7+h_8)\eta \\ + (-h_1-h_2-h_3-h_4+h_5+h_6+h_7+h_8)\zeta \,\right]$$ The interpolation with the dual octahedron of an arbitrary hexahedron is linear again.

At last, according to Wikipedia:
In all dimensions higher than four, there are only three convex regular polytopes: the simplex, the hypercube, and the cross-polytope. In three dimensions, these coincide with the tetrahedron, the cube, and the octahedron. Where the (self-dual, linear) simplex may be considered as a trivial case.

Question. In all dimensions higher than three, does the isoparametric transformation give a linear interpolation for an arbitrary function discretized at the dual polytope of a "finite element" with the hypercube or the cross-polytope as a parent?

Due to overwhelming lack of interest I've received a Tumbleweed badge for this question. Therefore I'm trying to answer it myself. The question will be referred to as the Dual = Linear conjecture . Disclaimer: the following is only a partial answer, because a general proof for dual polytopes in a space of dimension $N$ is still lacking.
The polytopes are defined in a space of dimension $N$. The (isoparametric) Cartesian coordinates in this space are $(c_1,c_2, \cdots ,c_N)$ . Input for Maple is generated by a Pascal program, i.e. by employing brute force Computer Algebra. Sample Maple input is shown for three dimensional space $(N=3)$. We have two cases:

• the HyperCube as the dual polytope of the PolyStar
• the PolyStar as the dual polytope of the HyperCube

First case: the HyperCube as the dual polytope of the PolyStar. The following steps have been taken in order to obtain an answer.

1. Define the $(2N+1)$ shape functions $S_k$ of the (normed) PolyStar:
S0 := 1-c1^2-c2^2-c3^2;
S1 := c1*(c1-1)/2;
S2 := c1*(c1+1)/2;
S3 := c2*(c2-1)/2;
S4 := c2*(c2+1)/2;
S5 := c3*(c3-1)/2;
S6 := c3*(c3+1)/2;

2. Define the interpolation $g$ of the function values $g_k$ at the vertices of the PolyStar:
g := g0*S0+g1*S1+g2*S2+g3*S3+g4*S4+g5*S5+g6*S6;

3. Evaluate the interpolation $g$ at the vertices of the dual element of the PolyStar, being the (normed) HyperCube, resulting in values $f_k$:
f1 := eval(g,{c1=-1,c2=-1,c3=-1});
f2 := eval(g,{c1=+1,c2=-1,c3=-1});
f3 := eval(g,{c1=-1,c2=+1,c3=-1});
f4 := eval(g,{c1=+1,c2=+1,c3=-1});
f5 := eval(g,{c1=-1,c2=-1,c3=+1});
f6 := eval(g,{c1=+1,c2=-1,c3=+1});
f7 := eval(g,{c1=-1,c2=+1,c3=+1});
f8 := eval(g,{c1=+1,c2=+1,c3=+1});

4. Define the $(2^N)$ shape functions $H_k$ and the function $f$ interpolating the values $f_k$ at the vertices of the dual (normed) HyperCube:
H1 := (1-c1)/2*(1-c2)/2*(1-c3)/2;
H2 := (1+c1)/2*(1-c2)/2*(1-c3)/2;
H3 := (1-c1)/2*(1+c2)/2*(1-c3)/2;
H4 := (1+c1)/2*(1+c2)/2*(1-c3)/2;
H5 := (1-c1)/2*(1-c2)/2*(1+c3)/2;
H6 := (1+c1)/2*(1-c2)/2*(1+c3)/2;
H7 := (1-c1)/2*(1+c2)/2*(1+c3)/2;
H8 := (1+c1)/2*(1+c2)/2*(1+c3)/2;
f := f1*H1+f2*H2+f3*H3+f4*H4+f5*H5+f6*H6+f7*H7+f8*H8;

5. At last, let Maple do some work:
sort(simplify(f));

Resulting in:$$-1/2\,{\it c1}\,{\it g1}+1/2\,{\it c1}\,{\it g2}-1/2\,{\it c2}\,{\it g3}+1/2\,{\it c2}\,{\it g4}-1/2\,{\it c3}\,{\it g5}+1/2\,{\it c3}\,{ \it g6}-2\,{\it g0}+1/2\,{\it g1}+1/2\,{\it g2}+1/2\,{\it g3}+1/2\,{ \it g4}+1/2\,{\it g5}+1/2\,{\it g6}$$ By visual inspection, we see that the result is indeed linear in the $c_k$ coordinates.

Second case: the PolyStar as the dual polytope of the HyperCube. The following steps have been taken in order to obtain an answer.

1. Define the $(2^N)$ shape functions $H_k$ of the (normed) Hypercube.
2. And the function $f$ interpolating the values $f_k$ at its vertices: see above.
3. Evaluate the interpolation $f$ at the vertices of the dual element of the HyperCube, which is the (normed) PolyStar, resulting in values $g_k$:
g0 := eval(f,{c1=0,c2=0,c3=0});
g1 := eval(f,{c1=-1,c2=0,c3=0});
g2 := eval(f,{c1=+1,c2=0,c3=0});
g3 := eval(f,{c1=0,c2=-1,c3=0});
g4 := eval(f,{c1=0,c2=+1,c3=0});
g5 := eval(f,{c1=0,c2=0,c3=-1});
g6 := eval(f,{c1=0,c2=0,c3=+1});

4. Define the $(2N+1)$ shape functions $S_k$ and the function $g$ interpolating the values $g_k$ at the vertices of the dual (normed) PolyStar: see above.
5. At last, let Maple do some work:
sort(simplify(g));

Resulting in:$$-1/8\,{\it c1}\,{\it f1}+1/8\,{\it c1}\,{\it f2}-1/8\,{\it c1}\,{\it f3}+1/8\,{\it c1}\,{\it f4}-1/8\,{\it c1}\,{\it f5}+1/8\,{\it c1}\,{ \it f6}-1/8\,{\it c1}\,{\it f7}+1/8\,{\it c1}\,{\it f8}-1/8\,{\it c2} \,{\it f1}-1/8\,{\it c2}\,{\it f2}+1/8\,{\it c2}\,{\it f3}+1/8\,{\it c2}\,{\it f4}-1/8\,{\it c2}\,{\it f5}-1/8\,{\it c2}\,{\it f6}+1/8\,{ \it c2}\,{\it f7}+1/8\,{\it c2}\,{\it f8}-1/8\,{\it c3}\,{\it f1}-1/8 \,{\it c3}\,{\it f2}-1/8\,{\it c3}\,{\it f3}-1/8\,{\it c3}\,{\it f4}+1 /8\,{\it c3}\,{\it f5}+1/8\,{\it c3}\,{\it f6}+1/8\,{\it c3}\,{\it f7} +1/8\,{\it c3}\,{\it f8}+1/8\,{\it f1}+1/8\,{\it f2}+1/8\,{\it f3}+1/8 \,{\it f4}+1/8\,{\it f5}+1/8\,{\it f6}+1/8\,{\it f7}+1/8\,{\it f8}$$ By visual inspection, we see that the result is again linear in the $c_k$ coordinates.

In this way, I've proved the Dual = Linear conjecture for spaces with dimension $N = 2,3,4,5,6,7,8$. Then Maple gives up. And me too, for the moment being.
Here is the program that generates the input for Maple:

program maple;
type punt = array of integer;
function Letterlijk(nr : integer) : string; { Convert Natural to String } const cijfer : string = '0123456789'; var no,t,tel,wel : integer; vgl : string; c : char; begin no := nr; tel := 0; { Analysis } vgl := ''; while no > 0 do begin c := cijfer[(no mod 10) + 1]; vgl := vgl + c; no := no div 10; tel := tel + 1; end; wel := tel div 2; for t := 0 to wel-1 do begin c := vgl[t+1]; vgl[t+1] := vgl[tel-t]; vgl[tel-t] := c; end; if vgl = '' then vgl := '0'; Letterlijk := vgl; end;
function twee(power : integer) : integer; { Compute 2^power } var k : integer; h : integer; begin h := 1; for k := 1 to power do h := h shl 1; twee := h; end;
function crd_H(nr,N : integer) : punt; { Node Number -> Hypercube Vertex Coordinates } var P : punt; h,k : integer; begin h := nr-1; SetLength(P,N); for k := 1 to N do begin P[k-1] := 2*(h and 1)-1; h := h shr 1; end; crd_H := P; end;
function crd_S(nr,N : integer) : punt; { Node Number -> PolyStar Vertex Coordinates } var P : punt; b,v,k : integer; begin SetLength(P,N); for k := 1 to N do begin P[k-1] := 0; end; if nr > 0 then begin b := (nr-1) div 2; v := (nr-1) mod 2; P[b] := 2*v-1; end; crd_S := P; end;
procedure shapes_H(N : integer); { HyperCube Shape Functions } var k,h,i,D : integer; c,s : string; begin D := twee(N); for i := 1 to D do begin h := i-1; s := ' := '; for k := 0 to N-1 do begin c := Letterlijk(k+1); if (h and 1) = 0 then s := s + '(1-c' + c + ')/2*' else s := s + '(1+c' + c + ')/2*'; h := h shr 1; end; Writeln('H',i,Copy(s,1,Length(s)-1),';'); end; s := ''; for k := 1 to D do begin c := Letterlijk(k); s := s + 'f' + c + '*H' + c + '+'; end; Writeln('f := ',Copy(s,1,Length(s)-1),';'); end;
procedure shapes_S(N : integer); { PolyStar Shape Functions } var k,i : integer; c,s : string; begin s := ' := 1'; for k := 1 to N do begin c := Letterlijk(k); s := s + '-c' + c + '^2'; end; s := s + ';'; Writeln('S0',s); for i := 1 to 2*N do begin s := ' := '; c := Letterlijk((i+1) div 2); if (i mod 2) = 1 then s := s + 'c' + c + '(c' + c + '-1)/2;' else s := s + 'c' + c + '(c' + c + '+1)/2;'; Writeln('S',i,s); end; s := 'g := '; for k := 0 to 2*N do begin c := Letterlijk(k); s := s + 'g' + c + '*S' + c + '+'; end; Writeln(Copy(s,1,Length(s)-1),';'); end;
procedure test_H(N : integer); { Dual = Linear test for HyperCube in PolyStar } var k,D,i : integer; s,c : string; P : punt; begin shapes_S(N);
SetLength(P,N); D := twee(N); for k := 1 to D do begin c := Letterlijk(k); s := 'f' + c + ' := eval(g,{'; P := crd_H(k,N); for i := 1 to N do begin if P[i-1] = -1 then c := '-1'; if P[i-1] = +1 then c := '+1'; s := s + 'c' + Letterlijk(i) + '=' + c + ','; end; c := Letterlijk(N); Writeln(Copy(s,1,Length(s)-1),'});'); end;
shapes_H(N); Writeln('sort(simplify(f));'); end;
procedure test_S( N : integer); { Dual = Linear test for PolyStar in HyperCube } var k,i : integer; s,c : string; P : punt; begin shapes_H(N);
s := 'g0 := eval(f,{'; for i := 1 to N do s := s + 'c' + Letterlijk(i) + '=0,'; Writeln(Copy(s,1,Length(s)-1),'});'); SetLength(P,0); for k := 1 to 2*N do begin c := Letterlijk(k); s := 'g' + c + ' := eval(f,{'; P := crd_S(k,N); for i := 1 to N do begin if P[i-1] = 0 then c := '0'; if P[i-1] = -1 then c := '-1'; if P[i-1] = +1 then c := '+1'; s := s + 'c' + Letterlijk(i) + '=' + c + ','; end; c := Letterlijk(N); Writeln(Copy(s,1,Length(s)-1),'});'); end;
shapes_S(N); Writeln('sort(simplify(g));'); end;
begin { test_H(8); } { test_S(9); } test_H(3); Writeln; test_S(3); end.