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Examples with data involving a Dirac function

4.7 General Algorithm and Numerical Experiments

4.7.3 Examples with data involving a Dirac function

The first problem with a Dirac function on the right hand side reads as ( detD2u=πδ(1/2,1/2) inΩ,

u= q

(x−0.5)2

y−0.5¢2

on∂Ω, (4.41)

The exact solutionuof problem (4.41) is defined by u(x,y)=

q

(x−0.5)2

y−0.5¢2

∀(x,y)∈Ω¯,

As in subsection 4.5.4 for both solvers we approximate the Dirac functionδ(α,β) by f(α,β)

ε ¡

x,y¢

= ε2

π³

ε2+(x−α)2

yβ¢2´2 ,

whereεis a small positive value. Therefore, we approximateπδ(1/2,1/2)byfε(1/2,1/2)(x,y). To estimate the error norms, we assume that lim

h0lim

ε→0uηε=u. Moreover, as in subsection 4.5.4, we setε=10−3/2.

In this experiment, we expect the adaptive algorithm to refine the mesh around (0.5, 0.5), i.e., where the Dirac function occurs. Figure 4.12 shows indeed the refined meshes for both solvers and differentT OL.

Table 4.16 shows numerical results obtained by PMA and LSMA solvers. For both solvers, when T OLdecreases,hmi n,hmax,ηIK,L2error norm decrease, and the number of elements and nodes increase. For PMA, the number of elements and nodes is larger than LSMA. — Figure 4.13 displays the refined meshes for LSMA with fixedT OL=0.25 and different values ofε. In this figure, we observe that the smaller the parameterεis, the larger the number of elements.

More precisely, Table 4.17 shows that asεdecreases, thehmi n,hmax,L2error norm decreases and the number of elements and nodes increases. Asεgets smaller, the problem becomes more challenging, and therefore,ηIK increases.

In this problem we consider two Dirac function on the right hand side ( detD2u=π2δ(1/4,1/2)+π2δ(3/4,1/2) inΩ,

u=g on∂Ω, (4.42)

T OL=0.5 T OL=0.25 T OL=0.125

T OL=0.5 T OL=0.25 T OL=0.125

Figure 4.12 – Examples with Dirac function for data f(x,y) = πδ(1/2,1/2) and g(x,y) = q

(x−0.5)2

y−0.5¢2

. Graphs of the final adapted mesh for various values ofT OL and fixedε=103/2. Top row: Graphs obtained with PMA solver after 800 timesteps. Bottom row:

Graphs obtained with LSMA solver after 500 iterations.

ε=10−3/2 ε=10−2 ε=10−5/2

Figure 4.13 – Examples with Dirac function for data f(x,y) = πδ(1/2,1/2) and g(x,y) = q

(x−0.5)2

y−0.5¢2

. Graphs of the final adapted mesh for various values ofεand fixed T OL=0.25. The graphs obtained with LSMA solver after 500 iterations.

4.7. General Algorithm and Numerical Experiments

Table 4.16 – Examples with Dirac function for data f(x,y) = πδ(1/2,1/2) and g(x,y) = q

(x−0.5)2

y−0.5¢2

. Convergence behavior of the algorithm for various values of parame-terT OLand fixedε=103/2. The columns contain the final minimal and maximal mesh size, the final numbers of elements and nodes, the value of the estimator, andL2error norms. Top table: Results obtained with PMA solver after 800 timesteps. Bottom table: Results obtained with LSMA solver after 500 iterations.

PMA

T OL hmi n hmax # elem # nodes ηI ||u−uh||L2

0.5 1.21e-02 1.90e-01 627 328 7.24e-01 3.71e-02 0.25 7.85e-03 1.23e-01 1331 739 5.23e-01 1.37e-02 0.125 3.58e-03 5.57e-02 3425 1748 3.50e-01 6.68e-03

LSMA

T OL hmi n hmax # elem # nodes ηI ||u−uh||L2

0.5 1.36e-02 1.77e-01 509 269 8.71e-01 7.73e-02 0.25 8.41e-03 1.24e-01 1128 582 5.50e-01 3.84e-02 0.125 4.89e-03 7.78e-02 2920 1489 3.70e-01 1.92e-02

Table 4.17 – Examples with Dirac function for data f(x,y) = πδ(1/2,1/2) and g(x,y) = q

(x−0.5)2

y−0.5¢2

. Convergence behavior of the algorithm for various values of param-eterε=10−3/2and fixedT OL=0.25. The columns contain the final minimal and maximal mesh size, the final numbers of elements and nodes, the value of the estimator, andL2error norms. The results obtained with LSMA solver after 500 iterations.

LSMA ε2 hmi n hmax # elem # nodes ηI ||u−uh||L2

1e-03 8.41e-03 1.24e-01 1128 582 5.50e-01 3.84e-02 1e-04 7.93e-03 1.40e-01 1224 636 0.12e-01 2.29e-02 1e-05 4.19e-03 7.02e-02 2986 1544 0.15e-01 1.29e-02

where error norms, we assume that lim

h→0lim

ε→0uηε=u. Moreover, as before we setε=10−3/2.

In this experiment, we expect the adaptive algorithm to refine the mesh around the

neighbor-hood of the line between (1/4, 1/2) and (3/4, 1/2). Figure 4.12 displays the refined meshes for both solvers and differentT OL, which shows the expected behavior.

Table 4.16 shows numerical results obtained by PMA and LSMA solvers. For the PMA, although theL2error norm is accurate to the order of 10−3, it does not decrease withT OL. On the other hand, for LSMA, theL2error norm is of order 102, and it decreases monotonically withT OL.

T OL=0.5 T OL=0.25 T OL=0.125

T OL=0.5 T OL=0.25 T OL=0.125

Figure 4.14 – Examples with Dirac function for data f(x,y)=π2δ(1/4,1/2)+π2δ(3/4,1/2)andg(x,y) is giver in (4.43). Graphs of the final adapted mesh for various values ofT OL and fixed ε=10−3/2. Top row: Graphs obtained with PMA solver after 800 timesteps. Bottom row:

Graphs obtained with LSMA solver after 500 iterations.

4.7. General Algorithm and Numerical Experiments

Table 4.18 – Examples with Dirac function for dataf(x,y)=π2δ(1/4,1/2)+π2δ(3/4,1/2)andg(x,y) is giver in (4.43). Convergence behavior of the algorithm for various values of parameterT OL and fixedε=10−3/2. The columns contain the final minimal and maximal mesh size, the final numbers of elements and nodes, the value of the estimator, andL2error norms. Top table:

Results obtained with PMA solver after 800 timesteps. Bottom table: Results obtained with LSMA solver after 500 iterations.

PMA

T OL hmi n hmax # elem # nodes ηI ||uuh||L2

0.5 1.11e-02 3.27e-01 722 373 7.72e-01 7.75e-03 0.25 5.96e-03 2.00e-01 1886 969 4.85e-01 7.71e-03 0.125 3.96e-03 1.26e-02 4664 2371 2.85e-01 8.42e-03

LSMA

T OL hmi n hmax # elem # nodes ηI ||u−uh||L2

0.5 1.23e-02 2.17e-01 687 356 5.98e-01 3.84e-02 0.25 8.14e-03 2.25e-01 1617 828 4.82e-01 2.08e-02 0.125 4.89e-03 7.78e-02 2920 1489 3.70e-01 1.92e-02

5 Conclusion

In this thesis, numerical methods for solutions of first and second-order fully nonlinear equations have been presented. We have discussed an operator-splitting/ finite element method for the numerical solution of the Dirichlet problem for orthogonal maps, and we have extended it by introducing an anisotropic space adaptivity method. Then, we have derived least-squares/relaxation methods for the numerical solutions of Jacobian equation/inequality, and the three dimensional Monge-Ampère equation. We have also developed a second order time integration method for the approximation of a parabolic and elliptic 2D Monge-Ampère equation. Lastly, we have introduced an isotropic adaptive method for the two dimensional elliptic Monge-Ampère equation.

In Chapter 1, we tackled the orthogonal map equation and we proposed an operator-splitting/

finite element method for the numerical solution of the Dirichlet problem. This method is based on a variational principle, the introduction of the associated flow problem, and a time-stepping splitting algorithm. Results show the robustness and the flexibility of this method and its ability to approximate solutions with line singularities on convex domains, with convergence of order one for theL2(Ω) norm of the approximation error. Then the method has been extended by proposing an anisotropic adaptive mesh algorithm that allows to track the singularities of the gradient of the solution more accurately. Numerical experiments have shown the robustness of the derived estimator and the adaptive algorithm.

In Chapter 2, we focused on the numerical approximation of the solutions of the prescribed Ja-cobian equation and inequality. The proposed method relies on a least-squares reformulation of the original problem and on a relaxation algorithm. The method is based on variational approaches where the use of relaxation algorithm allows the decoupling of the least-squares problem into local nonlinear and to variational problems. We have presented the implemen-tation in the finite element space and numerical examples to validate the method.

In Chapter 3, a least-squares/relaxation finite element method also has been introduced to solve the Dirichlet problem for the three-dimensional elliptic Monge-Ampère equation.

The method resembles the least squares framework that has been developed for several fully

nonlinear equations, and in Chapter 2. In this case, new solvers have been derived to provide solutions for the local nonlinear and variational problems. Numerical results have been validated by various test cases usingP1andQ1polynomials. The method achieves nearly optimal orders for theL2(Ω) andH1(Ω) error norms when smooth solutions are approximated.

In Chapter 4, we presented a numerical method for the solution of a parabolic 2D Monge-Ampère equation. Moreover, we have shown the efficiency of the method in capturing station-ary solutions, such as the solutions of the elliptic Monge-Ampère equation. In the numerical examples, when tested to smooth solutions, the method had nearly optimal orders for the L2(Ω) andH1(Ω) error norms. The method proved to be also robust at approximating non-smooth solutions. We have derived a heuristic error estimate, and we have designed an adaptive algorithm for the two-dimensional elliptic Monge-Ampère equation. Numerical experiments confirmed the robustness of the estimators and adaptive algorithm by using two different solvers.

It is possible to extent this work in different directions by working further on the following subtopics. For example, the operator-splitting/finite element method that used to solve the orthogonal maps can be broadened to include the rigid maps problems. The prescribed Jacobian equation can be extended to include a transformation map of the formu:Ω⊂R3→ R3. This will allow us to tackle problems in optimal transportation in 3 dimensional setting.

The parabolic and elliptic Monge-Ampère equation can also be extended to solve not only applications in optimal transport, but also financial ones. Last, the derived error estimates and the adaptive algorithms that are essentially designed for orthogonal maps and Monge-Ampère equation can be improved and extended to include other fully nonlinear equations such as Eikonal and Pucci equations.

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