The set of individual source signals, s ( t ) = ( s 1 ( t ) , … … , s n ( t ) ) T , is 'mixed' using a matrix, A = [ a i j ] ∈ ∈ R m × × n , to produce a set of 'mixed' signals, x ( t ) = ( x 1 ( t ) , … … , x m ( t ) ) T , as follows. Usually, n is equal to m . If m > n , then the system of equations is overdetermined and thus can be unmixed using a conventional linear method. If n > m , the system is underdetermined and a non-linear method must be employed to recover the unmixed signals. The signals themselves can be multidimensional.