@Article{PU06,
author  = {Hartmut Prautzsch and Georg Umlauf},
title   = {Parametrizations for trinagular {$G^k$} spline surfaces of low degree}, 
journal = {ACM Transactions on Graphics},
year    = 2006,
volume  = 24,
number  = 4,
pages   = {1281-1293}
}