The identity matrix is the matrix for which a_ij = 1 when i = j, and 0 everywhere else. For example, in three dimensions, I = [1 0 0 ; 0 1 0 ; 0 0 1]. Here is a column vector: [0 1 0]'. x \mapsto y_1...y_n 1/3 1 + 1 = 2 1 \leq 3 - 1 \leq 5