MathDB

Find all functions f: \mathbb{R}^{ \plus{} }\to\mathbb{R}^{ \plus{} } satisfying f\left(x \plus{} f\left(y\right)\right) \equal{} f\left(x \plus{} y\right) \plus{} f\left(y\right) for all pairs of positive reals

x

and

y

. Here, \mathbb{R}^{ \plus{} } denotes the set of all positive reals.
Proposed by Paisan Nakmahachalasint, Thailand

Problem Statement