Functional analysis: are there no true functions with real values ​​of two or more variables?

I see the following theorem somewhere, without complete proof.

For each irrational number $ lambda $, there are continuous functions $ phi_k: (0,1) to mathbb R, k = 1,2,3,4,5 $, so that for all continuous functions $ f: (0,1) ^ 2 to mathbb R $, there is a continuous function $ g: (0,1) a mathbb R $such that $ f (x, y) = sum_ {k = 1} ^ 5 g ( phi_k (x) + lambda phi_k (y)) $.

There are some mysterious things about this result. Where does the number come from? $ 5 $ come and why $ lambda notin mathbb Q $?

Could anyone offer a reference for proof of this?