$2$-arcs of maximal size in the affine and the projective Hjelmslev   plane over $\mathbb{Z}_{25}$
  It is shown that the maximal size of a $2$-arc in the projective Hjelmslev plane over $\mathbb{Z}_{25}$ is $21$, and the $(21,2)$-arc is unique up to isomorphism. Furthermore, all maximal $(20,2)$-arcs in the affine Hjelmslev plane over $\mathbb{Z}_{25}$ are classified up to isomorphism. 
Michael Kiermaier, Matthias Koch and Sascha Kurz
math.CO