and we know it was encoded by multiplication by the above matrix A = ( 3 2
1 11 ). Since
encoding is E(v) = Av, decoding is achieved by multiplication by the inverse of A: D(v) =
A−1v. When is a 2 × 2 matrix invertible? In a first linear algebra course you learn that
a (square) matrix with real entries is invertible precisely when its determinant is nonzero,
and in the 2 × 2 case ( a b
c d ) has inverse