The method of Tur\'an in establishing the normal order for the number of prime divisors of a number is used to show that a certain class of arithmetic functions do not have a normal order.
On a generalization of arithmetic functions and the Ramanujan sums
On two conjectures of Sierpiński concerning the arithmetic functions $σ$ and $φ$
Ramanujan-Fourier series of certain arithmetic functions of two variables
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