Integral Equations : A querry

Dear Members,

I seek your assistance to understand the following:

When we combine EFIE and MFIE to form CFIE for a PEC formulation, why do we apply [n x ] to EFIE before adding it to MFIE. I heard that it is related to the stability of the solution, but I could not find any explanation in any book or literature. Can anyone suggest?

The similar question applies for not applying [n x] to MFIE in PEC formulation, however for dielectric formulations we apply [n x] to MFIE operator as well to the testing functions....

I shall be thankful if someone can suggest me the most suitable text so that i may read and learn.

If it is concerned with eigenvalues to the operator, I would like to read and solve some formulations to learn it, however i need a source of information.

regards,