The paper studies spectral sets of elements of Banach algebras as the zeros of holomorphic functions and describes them in terms of existence of idempotents. A new decomposition theorem characterizing spectral sets is obtained for bounded linear operators.

Let ${\left\{\beta \left(n\right)\right\}}_{n=0}^{\infty }$ be a sequence of positive numbers and $1\le p<\infty$. We consider the space $H^p\left(\beta \right)$ of all power series $f\left(z\right)={\sum }_{n=0}^{\infty }\widehat{f}\left(n\right)z^n$ such that ${\sum }_{n=0}^{\infty }{\left|\widehat{f}\left(n\right)\right|}^p\beta {\left(n\right)}^p<\infty$. We investigate strict cyclicity of $H_p^{\infty }\left(\beta \right)$, the weakly closed algebra generated by the operator of multiplication by $z$ acting on $H^p\left(\beta \right)$, and determine the maximal ideal space, the dual space and the reflexivity of the algebra $H_p^{\infty }\left(\beta \right)$. We also give a necessary condition for a composition operator to be bounded on $H^p\left(\beta \right)$ when $H_p^{\infty }\left(\beta \right)$ is strictly cyclic.