sentences:  We introduce jump formula_48 and average formula_49
  and
  The linear forms formula_51 and formula_52 are
  and
  
  The penalty parameter formula_55 is a positive constant.Increasing its value will reduce the jumps in the discontinuous solution.The term formula_56 is chosen to be equal to formula_57 for the symmetric interior penalty Galerkin method; it is equal to formula_58 for the non-symmetric interior penalty Galerkin method.