A question on Robinson's book

Question

I have a question on the proof of the statement 3.2.8 in Robinson's book ( A course in theory of groups) Robinson's book . On the first part of the proof, A is $n \times n$ diagonal block matrix and every block $a_i I_{n_i},i=1,...,k$ is of degree 1 x 1. Here I don't know what do $ I_{n_i}$ means?. I am a little bit confused on subscript of the identity.

proof of statement 3.2.8

Answer 1

$I_{n_i}$ is simply the $n_i \times n_i$ identity matrix.