Antispecial cycles on the Drinfeld upper half plane and degenerate   Hirzebruch-Zagier cycles
  We define the notion of antispecial cycles on the Drinfeld upper half plane in analogy to the notion of special cycles defined by Kudla and Rapoport in their Inventiones paper. We determine equations for antispecial cycles and calculate the intersection multiplicity of two antispecial cycles. The result is applied to calculate the intersection multiplicity of certain degenerate Hirzebruch-Zagier cycles. Finally we compare this intersection multiplicity to certain representation densities. 
Ulrich Terstiege