def _expand(self, **hints):
     n = self.order
     z = self.argument
     return (-1)**(n + 1) * \
            (fn(-n - 1, z) * sin(z) + (-1)**(-n) * fn(n, z) * cos(z))

 def _expand(self):
     n = self.order
     z = self.argument
     return fn(n, z) * sin(z) + (-1)**(n+1) * fn(-n-1, z) * cos(z)

def _jn(n, z):
    return fn(n, z)*sin(z) + (-1)**(n + 1)*fn(-n - 1, z)*cos(z)

def _yn(n, z):
    # (-1)**(n + 1) * _jn(-n - 1, z)
    return (-1)**(n + 1) * fn(-n - 1, z)*sin(z) - fn(n, z)*cos(z)

 def g(self, nu, z):
     if nu.is_integer:
         return fn(self, nu, z)