Generalising Wigner's theorem
  We analyse linear maps of operator algebras $\mathcal{B}_H(\mathcal{H})$ mapping the set of rank-$k$ projectors onto the set of rank-$l$ projectors surjectively. We give a complete characterisation of such maps for prime $n = \dim\mathcal{H}$. The solution is known for $k=l=1$ as the Wigner's theorem. 
Gniewomir Sarbicki, Dariusz Chru\'sci\'nski, Marek Mozrzymas
quant-ph math-ph math.MP math.OA