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}.
Comment: 15 pages, 4 figures. arXiv admin note: text overlap with arXiv:1607.04401, arXiv:1610.01500