Eigenvalues of Chebyshev differentiation matrix

Apply Dirichlet bc at $x=1$ , then we remove last column and last row of differentiation matrix.

#%config InlineBackend.figure_format='svg'
from pylab import *
from chebPy import *

figure(figsize=(8,8))
Nvalues = array([32,64,128,256])
lmax = []
for i in range(4):
    N = Nvalues[i]
    D, x = cheb(N)
    D = D[0:-1,0:-1]
    
    lam,_ = eig(D)
    lmax.append(abs(lam).max())
    subplot(2,2,i+1)
    plot(real(lam),imag(lam),".")
    grid(True),axis("equal"),title("N = "+str(N))

All the eigenvalues have non-positive real parts.

NOTE: Compare to Fig. 4.4-1 in Fornberg; the plot looks bit different for $N=64$ case.

loglog(Nvalues,lmax,"o-",label=r"$max|\lambda|$")
loglog(Nvalues,0.09*Nvalues**2,'--',label=r"$0.09 N^2$")
legend(), xlabel("N");

This shows that

\max_i |\lambda_i| = 0.09 N^2

(1)

3rd order Adams-Bashforth method¶

N = 50
D, x = cheb(N)
D = D[0:-1,0:-1]
lam,_ = eig(D)

dt = 7/N**2
plot(real(lam*dt),imag(lam*dt),"o",label="$dt=7/N^2$")

dt = 8/N**2
plot(real(lam*dt),imag(lam*dt),"*",label="$dt=8/N^2$")

# Adams-Bashforth
z = exp(1j*pi*arange(0,201)/100); r = z - 1
s = (23 - 16/z + 5/z**2)/12; rr = r/s; plot(real(rr),imag(rr))
axis('equal'); grid('on'); xlabel(r'$real(\lambda\Delta t)$')
ylabel(r'$imag(\lambda\Delta t)$')
title(r'Adams-Bashforth, N ='+str(N)), legend();

For $\Delta t = 7/N^2$ , all the values $\lambda\Delta t$ lie inside the stability domain of this Adams method. For $\Delta t = 8/N^2$ , some values are outside the stable region.