## L-shape Eigenvalue - 100 Digits - Computer Program

Robert Stephen Jones
October, 2015
www.hbeLabs.com
rsjones7 at yahoo dot com
The following three files can be used to calculate the lowest Dirichlet eigenvalue of the Laplacian within the famous L-shape (two unit-edged squares joined to adjacent edges of a third) to 100 digits. The corresponding eigenfunction is used to create the familiar MathWorks logo (matlab).

The Helmholtz equation is, $$\Delta\Psi({\mathbf r})+\lambda\Psi({\mathbf r}) = 0$$ and the boundary condition is $\Psi({\mathbf r})=0$ on the L-shape boundary.

This program is designed to convince the reader that this technique works.

• My computers are running Ubuntu linux, with pari/GP compiled with GMP and pthreads.
• The first few lines of program detM.gp is where you need to customize for your computer.
• In one terminal, run: "$gp lsh.gp" • In another terminal, run "$ tailf results.dat" to watch the output.
• There are comments in the routines.

### lsh.gp

This is the main program:

[download]

### detM.gp

This is a program that is spawned to calculate the point-matching matrix and its determinant:

[download]

### library.gp

This is a small, custom library file with a few subroutines:

[download]

### results.dat

This is what my computer output when run. The 50-digit value was reported within 2 minutes while it took another half an hour to reach 100 digits. See below for the meaning of the numbers.

[download]

### Notes

1. Example line:
 32 9.6397238440219410527094777 (-)   LOWER BOUND  (1.76 e-20) 19
At N=32, the determinant root "9.639..." decreased "(-)" compared to the previous one. This entry is effectively a "LOWER BOUND". The relative error between the (previous) upper bound and this lower bound is $\epsilon=1.76\times 10^{-20}$. There are 19 matching digits in these upper and lower bounds, "9.639723844021941052" The N=32 means there are 16 matching points (Chebyshev-like, see subroutine initialize) along each of two edges of one of the three squares.

2. Observe that for N>64, the values actually alternate, each providing one side of the next bound. When equally-spaced matching points are used instead, it will begin alternating from the beginning, but the convergence rate is a bit slower. I have confirmed this alternation up to about N=800, for both types of matching point distributions, which gives about 300 (equal-space) and 400 (Chebyshev) eigenvalue digits .

3. The calculation with N=192 is done simply to demonstrate that indeed the previous one is an upper bound.

4. If you want more digits, and faster, calculate a few digits, figure out the convergence rate, and jump to a higher N-value, perhaps picking up fifty digits on each iteration (not one). Repeat until you get as many digits as your computer will permit.

5. If you want this to work for you, make sure there is enough precision in the intermediate calculations to overcome the well-known "ill-conditioned" nature of this problem. The above program provides more than enough precision, and you can probably get by with a little less.

6. When I invoke the GP calculator, this is displayed:
                            GP/PARI CALCULATOR Version 2.7.3 (released)
amd64 running linux (x86-64/GMP-6.0.0 kernel) 64-bit version
compiled: Mar 20 2015, gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
threading engine: pthread
(readline v6.3 enabled, extended help enabled)

Copyright (C) 2000-2015 The PARI Group

PARI/GP is free software, covered by the GNU General Public License, and comes WITHOUT ANY WARRANTY
WHATSOEVER.

Type ? for help, \q to quit.
Type ?12 for how to get moral (and possibly technical) support.

parisize = 8000000, primelimit = 500000
?


www.hbeLabs.com
rsjones7 at yahoo dot com