We give all functions ƒ , E: ℕ → ℂ which satisfy the relation
for every a, b, c ∈ ℕ, where h ≥ 0 is an integers and K is a complex number. If n cannot be written as a2 + b2 + c2 + h for suitable a, b, c ∈ ℕ, then ƒ (n) is not determined. This is more complicated if we assume that ƒ and E are multiplicative functions.
Elliott, P. D. T. A. Probabilistic Number Theory I.. Grund. der Math. Wiss., 239, Springer-Verlag, New York, Berlin, 1979.
Grosswald, E. Representations of Integers as Sums of Squares. Springer, 2011.
Khanh, B. M. M. On conjecture concerning the functional equation. Annales Univ. Sci. Budapest., Sect. Comp. 46 (2017), 123—135.
Khanh, B. M. M. A note on a result of B. Bojan. Annales Univ. Sci. Budapest., Sect. Comp. 49 (2019), 285—297.
Khanh, B. M. M. On the equation f(n2+Dm2+k)=f(n)2+Df(m)2+k. Annales Univ. Sci. Budapest., Sect. Comp. 52 (2021), 217–241.
Park, Poo-Sung. Multiplicative function commutable with sums of squares. International Journal of Number Theory 14, 2 (2018), 469–478.
Park, Poo-Sung. On k-additive uniqueness of the set of squares for multiplicative functions. Aequationes mathematicae 92 (2018), 487–495.
Katái, I. and Phong, B. M. A characterization of functions using Lagrange’s Four-Square Theorem. Annales Univ. Sci. Budapest., Sect. Comp. 52 (2021), 177–185.
Katái, I. and Phong, B. M. Arithmetical functions commutable with sums of squares. Notes on Number Theory and Discrete Mathematics 27, 3 (2021), 143–154.