Let A be the generator of a cosine function C, t • R in a Banach space X; we shall connect the existence and uniqueness of a T-periodic mild solution of the equation u" = Au + f with the spectral property 1 • p(Cr) and, in case X is a Hilbert space, also with spectral properties of A.In [9] the following formula was established (~2 _ A) fo sinh ~(t -s)Csds = ((cosh (t -C,), t~R ,~e C , Spectral properties of cosine operator functions which directly implies 81 (*) # e p(Ct) ~ {(2; cosh (t = /~} c p(A) and sup I[((( 2 -A) -111 < + ~. ch ~t = ltO u r purpose in this paper is to investigate cases when this implication is an equivalence; in our considerations we are inspired by ideas of J. Priil3 in [10]. Namely, as in the case of semigroup generators, it turns out that the spectral condition 1 e p ( C r ) is equivalent to the existence and uniqueness of a T-periodic mild solution for the inhomogenous second order differential equation u" = Au + f, where f is a forcing periodic term (Theorem 1).In the case of a Hilbert space the results are completed by Theorem 2:AEQ. MATH.
$1 _,vk(t)dt = f~ AS1 _,Stuk(O)dt-'~ S1-,C, vk(O)dt + S1-t C,_sfk(S)ds dt LJO J