code: def backward(self, indexes, gy):
      (q,) = self.get_retained_inputs()
      return (gy[0] * -self._x * zeta(self._x + 1, q),)
  
file_path: chainer/chainer/functions/math/zeta.py
imports: from chainer.backends import cuda
  from chainer import function_node
  from chainer import utils
  from chainer.utils import type_check
label_desc: Use image node_id 6 for calling the Zeta obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: unknown