Totient summatory function
Totient summatory function
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Totient summatory function

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Totient summatory function

In number theory, the totient summatory function is a summatory function of Euler's totient function defined by

It is the number of ordered pairs of coprime integers (p,q), where 1 ≤ pqn.

The first few values are 0, 1, 2, 4, 6, 10, 12, 18, 22, 28, 32, ... (sequence A002088 in the OEIS). Values for powers of 10 are 1, 32, 3044, 304192, 30397486, 3039650754, ... (sequence A064018 in the OEIS).

Applying Möbius inversion to the totient function yields

where is the Möbius function. Φ(n) has the asymptotic expansion

where ζ(2) is the Riemann zeta function evaluated at 2, which is .

The summatory function of the reciprocal of the totient is

Edmund Landau showed in 1900 that this function has the asymptotic behavior

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