${\varphi }_n(z)$	nth principal mode
$S_n$	Modal mass of the nth mode
$m(z)$	Continuous mass along the z-axis
l	Total length of the shaft
$P(z)$	Distribution function of rotor unbalance
$P_s$	Unbalance equivalent of the sth-order mode shape.
$U_K$	Isolated unbalance at point K
$U_p$	Continuous unbalance of the rotor system
$q_n$	nth-order modal component of the unbalanced distribution
$\omega (t)$	Angular velocity of the power turbine rotor
$m_r$	Mass of the r-th disk
$e_r$	Eccentricity of the r-th disk
${\theta }_{ur}$	Initial unbalance azimuth of the r-th disk
$a(t)$	Angular acceleration of the power turbine rotor
$\alpha (t)$	Phase term
$\kappa (t)$	Rotational angle of the disk
${\omega }_{ws}$	Rated operating speed of the rotor
O	Order
f	Frequency
nrs	Rotating speed