Turán–Kubilius inequality
# Turán–Kubilius inequality

The Turán–Kubilius inequality is a mathematical theorem in probabilistic number theory. It is useful for proving results about the normal order of an arithmetic $function.^{:305–308}$ The theorem was proved in a special case in 1934 by Pál Turán and generalized in 1956 and 1964 by Jonas $Kubilius.^{:316}$

## Statement of the theorem

This formulation is from $Tenenbaum.^{:302}$ Other formulations are in $Narkiewicz^{:243}$ and in Cojocaru & $Murty.^{:45–46}$

Suppose f is an additive complex-valued arithmetic function, and write p for an arbitrary prime and ν for an arbitrary positive integer. Write

    $A(x)=\sum _{p^{\nu }\leq x}f(p^{\nu })p^{-\nu }(1-p^{-1})$

and

    $B(x)^{2}=\sum _{p^{\nu }\leq x}\left|f(p^{\nu })\right|^{2}p^{-\nu }.$

Then there is a function ε(x) that goes to zero when x goes to infinity, and such that for x ≥ 2 we have

    ${\frac {1}{x}}\sum _{n\leq x}|f(n)-A(x)|^{2}\leq (2+\varepsilon (x))B(x)^{2}.$

## Applications of the theorem

Turán developed the inequality to create a simpler proof of the Hardy–Ramanujan theorem about the normal order of the number ω(n) of distinct prime divisors of an integer $n.^{:316}$ There is an exposition of Turán's proof in Hardy & Wright, §22.11. $Tenenbaum^{:305–308}$ gives a proof of the Hardy–Ramanujan theorem using the Turán–Kubilius inequality and states without proof several other applications.

