@article {wit:ell.gen,
    author = {Witten, Edward},
     title = {Elliptic genera and quantum field theory},
   journal = {Communications in Mathematical Physics},
    volume = {109},
      year = {1987},
    number = {4},
     pages = {525--536},
  mrnumber = {MR0885560},
       doi = {10.1007/BF01208956},
}