Past exam of the mathematics course of the University of Cambridge / 2007 / iii / Paper 85 / 2 / b

Write $g(x)=\mu -x^2$ and $f_L(x)=-\mathrm{sgn}(x)g(x)$. Suppose first that $g^k(x)\ne 0$ for $0\le k<n$, so no critical hit occurs before the final iterate. Set $A_n=(-1{)}^n{\prod }_{k=0}^{n-1}\mathrm{sgn}(g^k(x))\in \{1,-1\}$. The case $n=1$ is the defining identity. If $f_L^n(x)=A_n{g}^n(x)$ and $g^n(x)\ne 0$, evenness of $g$ and oddness of $f_L$ away from zero giveInduction proves the sign lift of an even map formulaon noncritical orbit segments. A final zero value is harmless; using oddness at zero in a subsequent step is not.

The PDF's assertion that this holds for every $x$ is false with its stated $\mathrm{sgn}(0)=1$ convention. For $\mu >0$, choose $x=\sqrt{\mu }$. Then $g(x)=f_L(x)=0$, sobut the proposed right side for $n=2$ is $g^2(x)\mathrm{sgn}(x)\mathrm{sgn}(g(x))=+\mu$. This explicit counterexample identifies the missing noncritical qualification. An exact recurrence valid even at critical hits can instead be written with $y_k=g^k(x)$ and $f_L^k(x)=A_k{y}_k$: start with $A_0=1$, and updateThe second rule resets the sign because $f_L(0)=-g(0)$ independently of the previous sign. This proves the intended identity wherever it is valid and accounts for the exceptional critical itineraries without changing the map's definition.