Question 62
Required are all functions $f$ mapping non-negative reals to non-negative reals, fulfilling the identity
$$
f(x_1^2 + \cdots + x_n^2) = f(x_1)^2 + \cdots + f(x_n)^2
$$
for any choice of numbers $x_1, \dots, x_n$.
$$
f(x_1^2 + \cdots + x_n^2) = f(x_1)^2 + \cdots + f(x_n)^2
$$
for any choice of numbers $x_1, \dots, x_n$.