Consistent and inconsistent equations


In mathematics and particularly
in algebra, a linear or nonlinear system of equations is called consistent if there is at least one set of values for the unknowns that satisfies each equation in the system—that is, when substituted into each of the equations, they make each equation hold true as an identity. In contrast, a linear or non linear equation system is called inconsistent if there is no set of values for the unknowns that satisfies all of the equations.
If a system of equations is inconsistent, then it is possible to manipulate and combine the equations in such a way as to obtain contradictory information, such as 2 = 1, or x3 + y3 = 5 and x3 + y3 = 6.
Both types of equation system, consistent and inconsistent, can be any of overdetermined, underdetermined, or exactly determined.

Simple examples

Underdetermined and consistent

The system
has an infinite number of solutions, all of them having z = 1, and all of them therefore having x+y = 2 for any values of x and y.
The nonlinear system
has an infinitude of solutions, all involving
Since each of these systems has more than one solution, it is an indeterminate system.

Underdetermined and inconsistent

The system
has no solutions, as can be seen by subtracting the first equation from the second to obtain the impossible 0 = 1.
The nonlinear system
has no solutions, because if one equation is subtracted from the other we obtain the impossible 0 = 3.

Exactly determined and consistent

The system
has exactly one solution: x = 1, y = 2.
The nonlinear system
has the two solutions = and =, while
has an infinite number of solutions because the third equation is the first equation plus twice the second one and hence contains no independent information; thus any value of z can be chosen and values of x and y can be found to satisfy the first two equations.

Exactly determined and inconsistent

The system
has no solutions; the inconsistency can be seen by multiplying the first equation by 4 and subtracting the second equation to obtain the impossible 0 = 2.
Likewise,
is an inconsistent system because the first equation plus twice the second minus the third contains the contradiction 0 = 2.

Overdetermined and consistent

The system
has a solution, x = –1, y = 4, because the first two equations do not contradict each other and the third equation is redundant.
The system
has an infinitude of solutions since all three equations give the same information as each other. Any value of y is part of a solution, with the corresponding value of x being 7–2y.
The nonlinear system
has the three solutions =,, and.

Overdetermined and inconsistent

The system
is inconsistent because the last equation contradicts the information embedded in the first two, as seen by multiplying each of the first two through by 2 and summing them.
The system
is inconsistent because the sum of the first two equations contradicts the third one.

Criteria for consistency

As can be seen from the above examples, consistency versus inconsistency is a different issue from comparing the numbers of equations and unknowns.

Linear systems

A linear system is consistent if and only if its coefficient matrix has the same rank as does its augmented matrix.

Nonlinear systems