Estimate for Norm of a Composition Operator on the Hardy–Dirichlet Space

By using the Schur test, we give some upper and lower estimates on the norm of a composition operator on H2, the space of Dirichlet series with square summable coefficients, for the inducing symbol φ(s) = c1+ cqq-s where q≥ 2 is a fixed integer. We also give an estimate on the approximation numbers of such an operator.