How to interpret a formula from the handbook of applied cryptography In the handbook of applied cryptography, we can find the following formula for testing PRNG. My question is what does mean 2 in the upper index of the n_00 and others? It is not power because then the formula does not make sense.
The comment by @SEJPM is correct, all those subscripted $n_0,n_{01}$ etc are nonnegative integers counting the occurrences of subsequences such as $0,01$ respectively. 

So no problem with exponentiation.