Spectral decomposition and Gelfand's theorem

In this paper we are interested in spectral decomposition of an unbounded operator with discrete spectrum. We show that if $A$ generates a polynomially bounded $n$-times integrated group whose spectrum set $\sigma(A)={i\lambda_k; k\in$\mathbb{Z}$^* }$isdiscreteandsatisfies$\sum \frac{1}{|\lambda_k|^\ell\delta_k^n}<\infty$($n$and$\ell$ nonnegative integers), then there exists projectors $(P_k{)}_{k\in {\mathbb{Z}}^{\ast }}$ such that $\sum P_{k}x=x$ ($x\in D(A^{n+\mathrm{\ell }})$), where $\delta_k=\min(\frac{| \lambda_{k+1}-\lambda_k|}2, \frac{|\lambda_{k-1}-\lambda_k|}2)$.