Suppose, in a housing project, a sociologist passes out a questionnaire to the residents in which she asks each resident to list the other residents that they consider their best friends. For simplicity, we say that there are only 5 residents and their choices are summarized by the matrix:
The ones in each row indicate the choices of the person to the left of the row. For example, x chooses y and w as her best friends while y chooses x and z. Explain the moaning of the entries of the matrix AAt.