IMO 2019 Q1

 This problem from IMO 2019 (international maths olympiad) was surprisingly easier than expected.

Find all functions $f: Z \to Z$ such that

$$ f(2a) + 2f(b) = f(f(a+b)) \quad \quad \forall a,b \in Z$$

$Z$ is the set of integers.

A British Math Olympiad problem

 Find all non-negative integers $a, b$ such that

$$ \sqrt{a} + \sqrt{b} = \sqrt{2019}$$