# A complete surface with $K\to -\infty$

Does anyone know an example of a complete surface (certainly not necessarily embedded in $\Bbb R^3$) with negative curvature unbounded below? Any example I have computed so far is either not complete or has curvature bounded below. Any light that might be shed would be welcome. (Sadly, this corner of geometry is far from my expertise.)

• Can't you just take a rotationally symmetric example in polar coordinates? $ds^2 = dr^2 + f(r)^2\,d\theta^2$ where $f$ is a solution of the equation $f''(r)-(1+r^2)f(r) = 0$ with $f(0)=0$ and $f'(0)=1$? This will be complete with $K = -(1+r^2)<0$ (and $r$ is the distance from the pole $r=0$ in this metric). The explicit solution of the equation is $$f(r) = e^{r^2/2}\ \int_0^r e^{-\rho^2}\ d\rho$$. Commented Sep 21, 2013 at 10:27
• Robert, thanks so much. I was fixating on trying to get a singular metric at the origin, rather than going to infinity, and this was complicating my life a bit. Elegant that the Gaussian emerges yet again :) The question that's motivating this — have you any intuition — is whether as we go off to infinity (with $K\to -\infty$) there is a uniform bound (upper and lower) on $A(R)/R^2$ when $R$ is geodesic distance and $A(R)$ is the area of the geodesic ball of radius $R$. (Happy sabbatical!! More later!) Commented Sep 21, 2013 at 12:25
• @Ted Shifrin: You cannot expect to have a upper bound on $A(R)/R^2$: even the hyperbolic plane does not have one since $A(R)$ is exponential! However you do have the lower bound $A(R)/R^2 \ge \pi$ for any simply connected surface of non-positive curvature, by Günther-Bishop inequality. Commented Sep 21, 2013 at 13:00
• @BenoîtKloeckner: Sorry, I phrased it sloppily. I meant for small $R$ only, but I did not know Günther-Bishop. Thanks so much. As I said, I have never been enough of a Riemannian expert :) Much appreciated! Commented Sep 21, 2013 at 13:06
• @TedShifrin: As for estimating $A(R)/R^2$ for small $R$, there is the classical expansion $$A(R)/R^2 = \pi\left(1-\frac{K(p)}{12}\ R^2\right) + O(R^4),$$ where $A(R)$ is the area of the disc of radius $R$ centered on $p$. Commented Sep 21, 2013 at 14:05

show that a smooth function on a $\mathbb{R}^2$ is the curvature of a complete metric if and only if $\lim_{r\rightarrow \infty} \inf_{|p| > r} K(p) \leq 0.$ So, presumably this should give plenty of examples of the sort you seek, and maybe if you read the paper it discusses how you might construct such.