NIL geodesic triangles and their interior angle sums
  In this paper we study the interior angle sums of geodesic triangles in $\NIL$ geometry and prove that it can be larger, equal or less than $\pi$.   We use for the computations the projective model of $\NIL$ introduced by E. {Moln\'ar} in \cite{M97}. 
Jen\H{o} Szirmai
math.MG