# A typo in equation (6.24).

The closed loop transfer function ${\displaystyle G(s)\,}$ (which means ${\displaystyle H_{yr}(s)\,}$) should be ${\displaystyle {\frac {\omega _{0}^{2}}{s^{2}+2\zeta \omega _{0}s+\omega _{0}^{2}}}\,}$. The last term of the denominator is ${\displaystyle \omega _{0}^{2}\,}$ rather than ${\displaystyle \omega _{0}\,}$, and the second term is missing an additional ${\displaystyle s\,}$.