In control theory, the Routh-Hurwitz criterion is a necessary and sufficient criterion for proving the stability of LTI systems. For a characteristic polynomial (the denominator of the transfer function) of a stable system , the number of roots of must be equal to the number of changes in sign of the first column of the Routh array.

The Routh array is given by: Where the corresponding and terms are given by:

Interpretation

We have several cases for the elements in the first column:

  • No element is zero — then by definition, all the coefficients must be positive or all must be negative.
  • There’s a zero in the first column, but some other elements of the containing row are non-zero — we can in fact replace that 0 with an arbitrarily small positive number that can approach zero.
  • There’s a zero in the first column, and all other elements of the containing row are zero — in this case, we look at the row before the zero row, which contains the auxiliary polynomial . The auxiliary polynomial reveals the roots of .
  • Repeated roots of the characteristic equation on the -axis — if we have duplicated auxiliary polynomials, we have repeated roots on the -axis, implying the system is unstable.