http://www.numbertheory.org/book/mp103.pdf

On page 12 of the book I'm stumped.

It says that the 'column numbers in which the leading entries 1 occurs' are denoted by , where r is the number of nonzero rows.

They give an example, where r=3, which means there are 3 non-zero rows in the matrix (as far as I can tell). Then the give some values for the c's. I undersatnd how the obtained the first 3 values:

, , .

However, I don't see how they obtained values for and .