A bridge between U-frequent hypercyclicity and frequent hypercyclicity

Given $\mathcal{A}$ the family of weights $a=(a_n{)}_n$ decreasing to $0$ such that the series ${\sum }_{n=0}^{\mathrm{\infty }}{a}_n$ diverges, we show that the supremum on $\mathcal{A}$ of lower weighted densities coincides with the unweighted upper density and that the infimum on $\mathcal{A}$ of upper weighted densities coincides with the unweighted lower density. We then investigate the notions of $\mathcal{U}$-frequent hypercyclicity and frequent hypercyclicity associated to these weighted densities. We show that there exists an operator which is $\mathcal{U}$-frequently hypercyclic for each weight in $\mathcal{A}$ but not frequently hypercyclic, although the set of frequently hypercyclic vectors always coincides with the intersection of sets of $\mathcal{U}$-frequently hypercyclic vectors for each weight in $\mathcal{A}$.