We prove some versions of abstract Korovkin-type theorems in modular function spaces, with respect to filter convergence for linear positive operators, by considering several kinds of test functions. We give some results with respect to an axiomatic convergence, including almost convergence. An extension to non positive operators is also studied. Finally, we give some examples and applications to moment and bivariate Kantorovich-type operators, showing that our results are proper extensions of the...

For a sequence x ∈ ℓ₁∖c₀₀, one can consider the set E(x) of all subsums of the series ${\sum }_{n=1}^{\infty }x\left(n\right)$. Guthrie and Nymann proved that E(x) is one of the following types of sets: () a finite union of closed intervals; () homeomorphic to the Cantor set; homeomorphic to the set T of subsums of ${\sum }_{n=1}^{\infty }b\left(n\right)$ where b(2n-1) = 3/4ⁿ and b(2n) = 2/4ⁿ. Denote by ℐ, and the sets of all sequences x ∈ ℓ₁∖c₀₀ such that E(x) has the property (ℐ), () and ( ), respectively. We show that ℐ and are strongly -algebrable and is -lineable. We...

Zeta-generalized-Euler-constant functions, $$\gamma \left(s\right):=\sum _{k=1}^{\infty }\left(\frac{1}{k^s}-{\int }_k^{k+1}\frac{dx}{x^s}\right)$$ and $$\tilde{\gamma }\left(s\right):=\sum _{k=1}^{\infty }{\left(-1\right)}^{k+1}\left(\frac{1}{k^s}-{\int }_k^{k+1}\frac{dx}{x^s}\right)$$ defined on the closed interval [0, ∞), where γ(1) is the Euler-Mascheroni constant and $$\tilde{\gamma }$$ (1) = ln $$\frac{4}{\pi }$$ , are studied and estimated with high accuracy.

An asymptotic approximation of Wallis’ sequence W(n) = Πk=1n 4k 2/(4k 2 − 1) obtained on the base of Stirling’s factorial formula is presented. As a consequence, several accurate new estimates of Wallis’ ratios w(n) = Πk=1n(2k−1)/(2k) are given. Also, an asymptotic approximation of π in terms of Wallis’ sequence W(n) is obtained, together with several double inequalities such as, for example, $W\left(n\right)·(a_n+b_n)<\pi <W\left(n\right)·(a_n+b_n^{\text{'}})$ with $a_n=2+\frac{1}{2n+1}+\frac{2}{3{(2n+1)}^2}-\frac{1}{3n{(2n+1)}^{\text{'}}}{b}_n=\frac{2}{33{(n+1)}^{2^{\text{'}}}}{b}_n^{\text{'}}\frac{1}{13n^{2^{\text{'}}}}n\in \mathbb{N}$ .