Expanding interval map ($\inf |T^{\prime }|=\lambda >1$)

A piecewise smooth interval map is uniformly expanding when every smooth branch has derivative magnitude at least $\lambda >1$. A finite branch partition gives a symbolic itinerary and cylinders with diameters at most a constant times ${\lambda }^{-n}$. Expansion alone does not make every symbolic Gibbs measure absolutely continuous; the geometric potential and suitable branch regularity are essential.

For a sufficiently regular uniformly expanding full-branch interval map, the geometric potential assigns the inverse Jacobian determinant weight. Its transfer operator is $\mathcal{L}h(x)={\sum }_{Ty=x}h(y)/|T^{\prime }(y)|$. Under the standard smooth full-branch hypotheses, a positive Lipschitz fixed density $h$ generates the corresponding invariant symbolic Gibbs measure. A different potential generally produces another measure, possibly singular with respect to Lebesgue measure.

If cylinder weights are $\nu (C_w)={\int }_{{\Delta }_w}hdx$, $h\ge h_{\ast }>0$, and $h$ is Lipschitz with constant $L$, then $g_n$ is the logarithm of a cylinder average. For $x\in {\Delta }_w$, $|g_n-\log h(x)|\le L\mathrm{diam}({\Delta }_w)/h_{\ast }$. Thus uniformly shrinking expanding-map cylinders give the geometric convergence estimate. Conversely, if $g_n$ converge uniformly at that rate to $g$ and the symbolic law is shift invariant, $e^{g(E(x))}dx$ has exactly the same cylinder masses: subdivide a fixed cylinder into level-$n$ cylinders, compare integrals within factors $e^{\pm C{\lambda }^{-n}}$, and let $n$ increase. This proves absolute continuity and invariance.