Values of Arithmetical Functions Equal to a Sum of Two Squares
Let '(n) denote the Euler function. In this paper, we determine the order of growth for the number of positive integers n ≤ x for which '(n) is the sum of two square numbers. We also obtain similar results for the Dedekind function (n) and the sum of divisors function _(n).
Quart. J. Math. Oxford Ser., 56 (2005), 123-139.