
Let \(\varphi\) be a positive non-decreasing real valued function defined on \([0, \infty)\), and let \(f\) be any real valued function defined on \([0, \infty)\). We say that \(f\) is \(\varphi\)-slowly varying if \(\varphi (x)[f(x+ \alpha)-f(x)] \to 0\) as \(x \to \infty\) for each \(\alpha\). We say that \(f\) is uniformly \(\varphi\)-slowly varying if \(\sup \{\varphi (x) |f(x+ \alpha)-f(x)|: \alpha \in I \} \to 0\) as \(x \to \infty\) for every bounded interval \(I\). We state here five theorems that will be proved later in a longer communication. We also pose one question that seems to be difficult. Theorem 1. If \(f\) is \(\varphi\)-slowly varying and if \(\sum i/ \varphi (n)< \infty\), then \(f\) tends to a finite or infinite limit at \(\infty\). Theorem 2. If \(f\) is \(\varphi\)-slowly varying and measurable, then \(f\) is uniformly \(\varphi\)-slowly vayring. Theorem 3. Let \(f\) be \(\varphi\)-slowly varying and let \(\beta (x)= \sum ^\infty_{j=0} 1/ \varphi (x+j)\). If \(\varphi (x) \beta (x)\) is bounded, then \(f\) must be uniformly \(\varphi\)-slowly varying. Theorem 4. Suppose that \(\sum 1/ \varphi (n)< \infty\) and that \(\varphi (x+1)/ \varphi (x) \to 1\) as \(x \to \infty\). Then there exists a function \(f\) that is \(\varphi\)-slowly varying but not uniformly \(\varphi\)-slowly varying. Theorem 5. Let \(\beta (x)\) be the function of Theorem 4, and suppose that \(\varphi (x) \beta (x)\) is unbounded, but that \(\varphi (x) \beta (x)=o(x)\) as \(x \to \infty\). Then there exists a function \(f\) that is \(\varphi\)-slowly varying but not uniformly \(\varphi\)-slowly varying. Question. Does there exist a function \(f\) such that \(x[f(x+ \alpha)-f(x)] \to 0\) as \(x \to \infty\) for each \(\alpha\) but \(\sup \{|f(x+ \alpha)-f(x)|: \alpha \in [0,1] \} \nrightarrow 0\) as \(x \to \infty\)?
510.mathematics, Rate of growth of functions, orders of infinity, slowly varying functions, Article
510.mathematics, Rate of growth of functions, orders of infinity, slowly varying functions, Article
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