Given that for k2n (n=4 and k takes all integer values from 1 to n)
If we want to state the sum of all solutions is evenly divisible by n we write:
n
∑ k2n = 0 (mod n)
k=0
Is there an equally succinct way to state (in one sentence), that each solution taken individually, is evenly divisible by n?
If you mean all the squares are divisible by $n$, you can just say $\forall (k)k^2 \equiv 0 \pmod n$. Is that what you are looking for?