linear algebra – Identity between resolved and singular value density

I read the newspaper

Sengupta, Anirvan M. and Partha P. Mitra. "Distributions of the singular
values ​​for some random matrices. "Physical Review E 60.3 (1999): 3389.

but got stuck in equation (3):

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Right here $ lambda_n $ are the singular values ​​of a matrix $ M $ (possibly considered a random matrix), $ rho ( lambda) $ denotes the density of singular values ​​of $ M $, and the resolvent $ mathcal G (z) $ is defined by:

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So, how (3) does it come from (2)?