=+The characteristic equation associated to (P4.1.g) can be put in the form:

???????? = s1(????)s2(????) − c2 1(????)

s1(????)c2(????) − c1(????)s2(????) (P4.1.h)

or, in terms of trigonometric and hyperbolic functions:

???? = (1 + cos ???? cosh ????)

????(−cosh ???? sin ???? + cos ???? sinh ????) (P4.1.i)