$\nabla$

Gradient operator

${\nabla }^2$

Laplacian operator

$i$

Sqrt(−1)

$k_o$

Wave-number

$\widehat{E}$

Symbol of the operator

$\oplus$

Direct sum

$D=\frac{\partial }{\partial k}$

Derivatives in Sobolev space

$H^2$

Sobolev space with 2 derivatives

$R_{\lambda }\left(k\right)$

Resolvent

$P$

Projector

$G_{\lambda }\left(x-y\right)$

Green's function