code: def greenhall_sx(t, F, alpha):
      
      if F == float('inf'):
          return greenhall_sw(t, alpha+2)
      a = 2*greenhall_sw(t, alpha)
      b = greenhall_sw(t-1.0/float(F), alpha)
      c = greenhall_sw(t+1.0/float(F), alpha)
  
      return pow(F, 2)*(a-b-c)
docs: Eqn (8) from Greenhall2004