Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Discrete mathematics, continuous mathematics, and probability / 1 / d

Seek an affine function $f(x)=cx+d$. Its function composition with itself isEquating coefficients requires $c^2=a$ and $(c+1)d=b$. Because $a>0$ and $a\ne 1$, either choice $c=\sqrt{a}$ or $c=-\sqrt{a}$ has $c+1\ne 0$. Thus two valid functions areSubstitution into $c^{2}x+(c+1)d$ verifies each answer. These provide the requested maps; no assumption that every possible solution is affine function is needed.

Try a linear function $f(x)=cx$. The functional equation becomes $c^{2}x=(c+1)x$ for all $x$, so $c$ must be a root of a polynomial $c^2-c-1$. Its two roots of a polynomial giveFor either choice, $c^2=c+1$ proves the required equality directly. The positive coefficient is the golden ratio.