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We can check if l is an eigenvalue of A by
studying (l*identity_matrix(A.nrows())-A).rref().
More efficiently, we can directly check if l is an eigenvalue
of A
with (l*identity_matrix(A.nrows())-A).is_singular().
The command (l*identity_matrix(A.nrows())-A).right_nullity()
calculates the geometric multiplicity of an eigenvalue.