evs.py

from nutils import mesh, function, plot, _
import numpy as np
import scipy as sp


nelems = 10
xs = np.linspace(0, 1, nelems + 1)
ys = np.linspace(0, 1, nelems + 1)
domain, grid_geom = mesh.rectilinear([xs, ys])

basis = domain.basis('spline', degree=3)
geom = grid_geom

grad = basis.grad(geom)
outer = function.outer(grad, grad)
integrand = outer.sum(-1)
matrix = domain.integrate(integrand, geometry=geom, ischeme='gauss4')

cons = domain.boundary.project(1, onto=basis, geometry=geom, ischeme='gauss4')
I, = np.where(np.isnan(cons))

mx = matrix.toarray()
mx = mx[np.ix_(I, I)]
w, v = np.linalg.eigh(mx)

for i in range(len(I)):
    dd = np.zeros((len(basis),))
    dd[I] = v[:,i]
    bfun = basis.dot(dd)
    points, colors = domain.elem_eval([geom, bfun], ischeme='bezier9', separate=True)
    with plot.PyPlot('bfun', index=i) as plt:
        plt.mesh(points, colors)
        plt.colorbar()