The paper investigates linear functionals ϕ : ℝn → ℝ preserving a set ℳ⊆ ℝ, i.e., such that ϕ(v) ∈ ℳ for any vector u with components in ℳ. For various types of subsets real numbers, characterizations the preserve them are obtained. In particular, sets ℤ,ℚ, ℤ+,ℚ+,ℝ+, several infinite integers, bounded and unbounded intervals, finite numbers considered. A characterization also allows one to desc...

We prove an identity for basis functions of a general family of positive linear operators. It covers as special cases the Bernstein, Szász–Mirakjan and Baskakov operators. A corollary of our result can be considered a pointwise orthogonality relation. The Bernstein case is the univariate case of a remarkable identity which recently was presented by Jetter and Stöckler. As an application we give...

It is an open problem whether a separating operator acting between semiprime f-algebras weighted composition operator. In this respect, we have been largely motivated by the recent paper (Abid et al. in Positivity 21:521–537, 2017) which give positive answer when f-algebra domain unital. general case, prove that if and only almost contractive. As consequence, generalizes improve some well-known...