Totient summatory function

In number theory, the totient summatory function Φ ( n ) {\displaystyle \Phi (n)} is a summatory function of Euler's totient function defined by Φ ( n ) := ∑ k = 1 n φ ( k ) , n ∈ N . {\displaystyle \Phi (n):=\sum _{k=1}^{n}\varphi (k),\quad n\in \mathbb {N} .} It is the number of ordered pairs of coprime integers (p,q), where 1 ≤ p ≤ q ≤ n.