@article{GDZPPN002490021,
title = {The Circle Problem on Surfaces of Variable Negative Curvature.},
journal = {Monatshefte f\"ur Mathematik},
volume = {123},
year = {1997},
pages = {61-70},
zbl = {0876.58040},
url = {http://dml.mathdoc.fr/item/GDZPPN002490021}
}
Pollicott, Mark; Sharp, Richard. The Circle Problem on Surfaces of Variable Negative Curvature.. Monatshefte für Mathematik, Tome 123 (1997) pp. 61-70. http://gdmltest.u-ga.fr/item/GDZPPN002490021/