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We prove that, for a sequence of positive numbers δ(n), if $n^{1/2}\delta \left(n\right)\neg \to \infty$ as $n\to \infty$, to guarantee that the modified Szász-Mirakjan operators $S_{n,\delta }(f,x)$ converge to f(x) at every point, f must be identically zero.

We first give a necessary and sufficient condition for $x^{-\gamma }\varphi \left(x\right)\in L^p$, 1 < p < ∞, 1/p - 1 < γ < 1/p, where ϕ(x) is the sum of either ${\sum }_{k=1}^{\infty }{a}_{k}coskx$ or ${\sum }_{k=1}^{\infty }{b}_{k}sinkx$, under the condition that λₙ (where λₙ is aₙ or bₙ respectively) belongs to the class of so called Mean Value Bounded Variation Sequences (MVBVS). Then we discuss the relations among the Fourier coefficients λₙ and the sum function ϕ(x) under the condition that λₙ ∈ MVBVS, and deduce a sharp estimate for the weighted modulus of continuity of ϕ(x) in $L^p$ norm.