For the j {\displaystyle j} th amino acid, the values m ( j ) {\displaystyle m(j)} and f ( j ) {\displaystyle f(j)} are its mutability and frequency. The frequencies of the amino acids are normalised so that they sum to 1. If total number of occurrences of the j {\displaystyle j} th amino acid is n ( j ) {\displaystyle n(j)} , and N {\displaystyle N} is the total number of all amino acids, then