By allowing "u" to vary over all possible integers, an infinite family of solutions can be generated from a single solution ("x"1, "y"1). If the solutions are required to be "positive" integers ("x" > 0, "y" > 0), only a finite number of solutions may be possible. This restriction on the acceptable solutions allows some systems of Diophantine equations with more unknowns than equations to have a finite number of solutions; this is impossible for a system of linear equations when the solutions can be any real number (see Underdetermined system).