A Lattice Formulation of Super Yang-Mills Theories with Exact   Supersymmetry
  We construct SU($N$) super Yang-Mills theories with extended supersymmetry on hypercubic lattices of various dimensions keeping one or two supercharges exactly.   It is based on topological field theory formulation for the super Yang-Mills theories. Gauge fields are represented by compact unitary link variables, and the exact supercharges on the lattice are nilpotent up to gauge transformations. In particular, the lattice models are free from the vacuum degeneracy problem, which was encountered in earlier approaches. Thus, we do not need to introduce any supersymmetry breaking terms, and the exact supersymmetry is preserved wholly in the process of taking the continuum limit.   Among the models, we show that the desired continuum theories are obtained without any fine tuning of parameters for the cases ${\cal N}=2, 4, 8$ in two-dimensions. Also, the cases ${\cal N}=4, 8$ in three-dimensions are investigated, and a problem arising in four-dimensional models is discussed. 
Fumihiko Sugino (Okayama Inst. for Quantum Phys.)