# Inconsistent results when using boundary operator for maxwell's equations

Hello,

The results that I get when using the solver are inconsistent. I have the following two examples. I use the following code for the two examples:

``````import bempp.api
import numpy as np

# example 1: plate with four RWGs / triangles
vertices = np.array([[0, 0.004, 0.004, 0, 0.002],[0, 0, 0.004, 0.004, 0.002],[0,0,0,0,0]],dtype=np.float64)
elements = np.array([[0,1,2,3],[1,2,3,0],[4,4,4,4]],dtype=np.uint32)
grid = bempp.api.Grid(vertices, elements)

# example 2: one RWG / two triangles of the plate of example 1
#vertices = np.array(
#    [[0.0,0.004,0.004,0.002],[0.0,0.0,0.004,0.002],[0.0,0.0,0.0,0.0]],
#    dtype=np.float64)
#elements = np.array(
#    [[0,1],[1,2],[3,3]], dtype=np.uint32)
#grid = bempp.api.Grid(vertices, elements)

E0 = np.array([0.0, 1.0, 0.0]) # polarization of electric field
kv = np.array([0.0, 0, -314.1593]) # wave vector
k = np.linalg.norm(kv) # wave number

div_space = bempp.api.function_space(grid, "RWG", 0)
curl_space = bempp.api.function_space(grid, "SNC", 0)

def incident_field(x):
return E0*np.exp(1j*np.dot(kv,x))

@bempp.api.complex_callable
def tangential_trace(x, n, domain_index, result):
incident_field = E0*np.exp(1j*np.dot(kv,x))
result[:] = np.cross(incident_field, n)

trace_fun = bempp.api.GridFunction(div_space, fun=tangential_trace, dual_space=curl_space)

from bempp.api.operators.boundary import maxwell
elec = maxwell.electric_field(div_space, div_space, curl_space, k)

print(trace_fun._projections) # values of incident field term
z = elec.weak_form()
print(z._impl) # impedance matrix
``````

Example 1 contains a plate of 4 triangles / RWGs. This gives a 4x4 BEM matrix. Example 2 contains one RWG of the plate of example 1. The diagonal elements of the 4x4 BEM matrix in example 1 have the value:
1.44531462e-07+2.71073075e-06j
The single element BEM matrix in example 2 has the value:
1.60467455e-07+1.40209657e-05j
These values are not the same. I think the values of the diagonal elements of the BEM matrix in example 1 should be the same as the value of the single element of the BEM matrix in example 2.

I hope I formatted the code correctly.

Best regards,
Christiaan