Let $s\in \mathbf{C}$, $x\in \mathbf{R}/2\pi \mathbf{Z}$. We construct Dirichlet series $F(x,x)$ where for each fixed $s$ in a half plane, $\mathrm{Re}F(x,x)$, as a function of $x$, is a non-synthesizable absolutely convergent Fourier series. Because of the way the frequencies in $F$ are chosen, we are motivated to introduce a class of synthesizable absolutely convergent Fourier series which are defined in terms of idele characters. We solve the “problem of analytic continuation” in this setting by constructing pseudo-measures, determined by idele characters, when $\mathrm{Re}s\le 1$.

We study weighted $L_p$-integrability (1 ≤ p < ∞) of trigonometric series. It is shown how the integrability of a function with weight $x^{-\alpha }$ depends on some regularity conditions on Fourier coefficients. Criteria for the uniform convergence of trigonometric series in terms of their coefficients are also studied.

Integrability and $L^1-$convergence of modified cosine sums introduced by Rees and Stanojević under a class of generalized semi-convex null coefficients are studied by using Cesàro means of non-integral orders.

We show that, if the coefficients (an) in a series $a_0/2+{\sum }_{n=1}^{\infty }{a}_{n}cos\left(nt\right)$ tend to 0 as n → ∞ and satisfy the regularity condition that ${\sum }_{m=0}^{\infty }{{\sum }_{j=1}^{\infty }[{\sum }_{n=j{2}^m}^{(j+1)2^m-1}|a_n-a_{n+1}\left|\right]²}^{1/2}<\infty$, then the cosine series represents an integrable function on the interval [-π,π]. We also show that, if the coefficients (bn) in a series ${\sum }_{n=1}^{\infty }{b}_{n}sin\left(nt\right)$ tend to 0 and satisfy the corresponding regularity condition, then the sine series represents an integrable function on [-π,π] if and only if ${\sum }_{n=1}^{\infty }\left|b_n\right|/n<\infty$. These conclusions were previously known to hold under stronger restrictions on the sizes of the differences...