@InProceedings{PU99a,
author    = {Hartmut Prautzsch and Georg Umlauf},
title     = {Triangular $G^2$ splines},
booktitle = {Curve and Surface Design},
publisher = {Vanderbilt University Press},
year      = 1999,
editor    = {P.-L.~Laurent and P.~Sablonniere and L.L.~Schumaker},
pages     = {335-342}
}