The observer model of the physical system is then typically derived from the above equations. Additional terms may be included in order to ensure that, on receiving successive measured values of the plant's inputs and outputs, the model's state converges to that of the plant. In particular, the output of the observer may be subtracted from the output of the plant and then multiplied by a matrix formula_8; this is then added to the equations for the state of the observer to produce a so-called "Luenberger observer", defined by the equations below. Note that the variables of a state observer are commonly denoted by a "hat": formula_9 and formula_10 to distinguish them from the variables of the equations satisfied by the physical system.