1from chainer import backend
2from chainer import function_node
3from chainer import utils
4from chainer.utils import type_check
5
6
7class Arctanh(function_node.FunctionNode):
8
9    """Elementwise inverse hyperbolic tangent function."""
10
11    def check_type_forward(self, in_types):
12        type_check._argname(in_types, ('x',))
13        x_type, = in_types
14
15        type_check.expect(x_type.dtype.kind == 'f')
16
17    def forward(self, inputs):
18        self.retain_inputs((0,))
19        x, = inputs
20        xp = backend.get_array_module(x)
21        y = xp.arctanh(x)
22        return utils.force_array(y, dtype=x.dtype),
23
24    def backward(self, indexes, grad_outputs):
25        x, = self.get_retained_inputs()
26        gy, = grad_outputs
27        gx = 1. / (1 - x ** 2) * gy
28        return gx,
29
30
31def arctanh(x):
32    """Elementwise inverse hyperbolic tangent function.
33
34    Args:
35        x (:class:`~chainer.Variable` or :ref:`ndarray`): Input variable.
36
37    Returns:
38        ~chainer.Variable: Output variable.
39
40    """
41    return Arctanh().apply((x,))[0]
42