In this note we prove that if in observer equation

we choose matrix

in companion form, it is possible to give a very simple, explicit solution to unknown matrix

. It is furthermore shown that for almost all the matrices

, such that the pair (

) is reachable, matrix

is of full rank. All this allows us to establish efficient algorthims for designing reduced observers both for the estimate of state and for that of a given linear functional, even if the spectra of

and

are not disjointed.