# npdl.activations¶

Non-linear activation functions for artificial neurons.

A function used to transform the activation level of a unit (neuron) into an output signal. Typically, activation functions have a “squashing” effect. Together with the PSP function (which is applied first) this defines the unit type. Neural Networks supports a wide range of activation functions.

## Activations¶

 Activation() Base class for activations. Sigmoid() Sigmoid activation function. Tanh() Tanh activation function. ReLU() Rectify activation function. Linear() Linear activation function. Softmax() Softmax activation function. Elliot([steepness]) A fast approximation of sigmoid. SymmetricElliot([steepness]) Elliot symmetric sigmoid transfer function. SoftPlus() Softplus activation function. SoftSign() SoftSign activation function.

## Detailed Description¶

class npdl.activations.Activation[source][source]

Base class for activations.

derivative(input=None)[source][source]

Backward step.

Parameters: input : numpy.array, optional. If provide input, this function will not use last_forward.
forward(input)[source][source]

Forward Step.

Parameters: input : numpy.array the input matrix.
class npdl.activations.Sigmoid[source][source]

Sigmoid activation function.

derivative(input=None)[source][source]

The derivative of sigmoid is

$\begin{split}\frac{dy}{dx} & = (1-\varphi(x)) \otimes \varphi(x) \\ & = \frac{e^{-x}}{(1+e^{-x})^2} \\ & = \frac{e^x}{(1+e^x)^2}\end{split}$
Returns: float32 The derivative of sigmoid function.
forward(input, *args, **kwargs)[source][source]

A sigmoid function is a mathematical function having a characteristic “S”-shaped curve or sigmoid curve. Often, sigmoid function refers to the special case of the logistic function and defined by the formula $$\varphi(x) = \frac{1}{1 + e^{-x}}$$ (given the input $$x$$).

Parameters: input : float32 The activation (the summed, weighted input of a neuron). float32 in [0, 1] The output of the sigmoid function applied to the activation.


class Sigmoid(Activation):
"""Sigmoid activation function.
"""

def __init__(self):
super(Sigmoid, self).__init__()

def forward(self, input, *args, **kwargs):
"""A sigmoid function is a mathematical function having a
characteristic "S"-shaped curve or sigmoid curve. Often,
sigmoid function refers to the special case of the logistic
function and defined by the formula :math:\\varphi(x) = \\frac{1}{1 + e^{-x}}
(given the input :math:x).

Parameters
----------
input : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32 in [0, 1]
The output of the sigmoid function applied to the activation.
"""

self.last_forward = 1.0 / (1.0 + np.exp(-input))
return self.last_forward

def derivative(self, input=None):
"""The derivative of sigmoid is

.. math:: \\frac{dy}{dx} & = (1-\\varphi(x)) \\otimes \\varphi(x)  \\\\
& = \\frac{e^{-x}}{(1+e^{-x})^2} \\\\
& = \\frac{e^x}{(1+e^x)^2}

Returns
-------
float32
The derivative of sigmoid function.
"""
last_forward = self.forward(input) if input else self.last_forward
return np.multiply(last_forward, 1 - last_forward)


class npdl.activations.Tanh[source][source]

Tanh activation function.

The hyperbolic tangent function is an old mathematical function. It was first used in the work by L’Abbe Sauri (1774).

derivative(input=None)[source][source]

The derivative of tanh() functions is

$\begin{split}\frac{d}{dx} tanh(x) & = \frac{d}{dx} \frac{sinh(x)}{cosh(x)} \\ & = \frac{cosh(x) \frac{d}{dx}sinh(x) - sinh(x) \frac{d}{dx}cosh(x) }{ cosh^2(x)} \\ & = \frac{ cosh(x) cosh(x) - sinh(x) sinh(x) }{ cosh^2(x)} \\ & = 1 - tanh^2(x) \end{split}$
Returns: float32 The derivative of tanh function.
forward(input)[source][source]

This function is easily defined as the ratio between the hyperbolic sine and the cosine functions (or expanded, as the ratio of the half‐difference and half‐sum of two exponential functions in the points $$z$$ and $$-z$$):

$\begin{split}tanh(z) & = \frac{sinh(z)}{cosh(z)} \\ & = \frac{e^z - e^{-z}}{e^z + e^{-z}}\end{split}$

Fortunately, numpy provides tanh() methods. So in our implementation, we directly use $$\varphi(x) = \tanh(x)$$.

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 in [-1, 1] The output of the tanh function applied to the activation.


class Tanh(Activation):
"""Tanh activation function.

The hyperbolic tangent function is an old mathematical function.
It was first used in the work by L'Abbe Sauri (1774).
"""

def __init__(self):
super(Tanh, self).__init__()

def forward(self, input):
"""This function is easily defined as the ratio between the hyperbolic
sine and the cosine functions (or expanded, as the ratio of the
half‐difference and half‐sum of two exponential functions in the
points :math:z and :math:-z):

.. math:: tanh(z) & = \\frac{sinh(z)}{cosh(z)} \\\\
& = \\frac{e^z - e^{-z}}{e^z + e^{-z}}

Fortunately, numpy provides :meth:tanh methods. So in our implementation,
we directly use :math:\\varphi(x) = \\tanh(x).

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32 in [-1, 1]
The output of the tanh function applied to the activation.
"""
self.last_forward = np.tanh(input)
return self.last_forward

def derivative(self, input=None):
"""The derivative of :meth:tanh functions is

.. math:: \\frac{d}{dx} tanh(x) & = \\frac{d}{dx} \\frac{sinh(x)}{cosh(x)} \\\\
& = \\frac{cosh(x) \\frac{d}{dx}sinh(x) - sinh(x) \\frac{d}{dx}cosh(x) }{ cosh^2(x)} \\\\
& = \\frac{ cosh(x) cosh(x) - sinh(x) sinh(x) }{ cosh^2(x)}  \\\\
& = 1 - tanh^2(x)

Returns
-------
float32
The derivative of tanh function.
"""
last_forward = self.forward(input) if input else self.last_forward
return 1 - np.power(last_forward, 2)


class npdl.activations.ReLU[source][source]

Rectify activation function.

Two additional major benefits of ReLUs are sparsity and a reduced likelihood of vanishing gradient. But first recall the definition of a ReLU is $$h=max(0,a)$$ where $$a=Wx+b$$.

One major benefit is the reduced likelihood of the gradient to vanish. This arises when $$a>0$$. In this regime the gradient has a constant value. In contrast, the gradient of sigmoids becomes increasingly small as the absolute value of $$x$$ increases. The constant gradient of ReLUs results in faster learning.

The other benefit of ReLUs is sparsity. Sparsity arises when $$a≤0$$. The more such units that exist in a layer the more sparse the resulting representation. Sigmoids on the other hand are always likely to generate some non-zero value resulting in dense representations. Sparse representations seem to be more beneficial than dense representations.

derivative(input=None)[source][source]

The point-wise derivative for ReLU is $$\frac{dy}{dx} = 1$$, if $$x>0$$, or $$\frac{dy}{dx} = 0$$, if $$x<=0$$.

Returns: float32 The derivative of ReLU function.
forward(input)[source][source]

During the forward pass, it inhibits all inhibitions below some threshold $$ϵ$$, typically $$0$$. In other words, it computes point-wise

$y=max(0,x)$
Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the rectify function applied to the activation.


class ReLU(Activation):
"""Rectify activation function.

Two additional major benefits of ReLUs are sparsity and a reduced
likelihood of vanishing gradient. But first recall the definition
of a ReLU is :math:h=max(0,a) where :math:a=Wx+b.

One major benefit is the reduced likelihood of the gradient to vanish.
This arises when :math:a>0. In this regime the gradient has a constant value.
In contrast, the gradient of sigmoids becomes increasingly small as the
absolute value of :math:x increases. The constant gradient of ReLUs results in
faster learning.

The other benefit of ReLUs is sparsity. Sparsity arises when :math:a≤0.
The more such units that exist in a layer the more sparse the resulting
representation. Sigmoids on the other hand are always likely to generate
some non-zero value resulting in dense representations. Sparse representations
seem to be more beneficial than dense representations.
"""

def __init__(self):
super(ReLU, self).__init__()

def forward(self, input):
"""During the forward pass, it inhibits all inhibitions below some
threshold :math:ϵ, typically :math:0. In other words, it computes point-wise

.. math:: y=max(0,x)

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the rectify function applied to the activation.
"""
self.last_forward = input
return np.maximum(0.0, input)

def derivative(self, input=None):
"""The point-wise derivative for ReLU is :math:\\frac{dy}{dx} = 1, if
:math:x>0, or :math:\\frac{dy}{dx} = 0, if :math:x<=0.

Returns
-------
float32
The derivative of ReLU function.
"""
last_forward = input if input else self.last_forward
res = np.zeros(last_forward.shape, dtype=get_dtype())
res[last_forward > 0] = 1.
return res


class npdl.activations.Linear[source][source]

Linear activation function.

derivative(input=None)[source][source]

Backward propagation. The backward also return identity matrix.

Returns: float32 The derivative of linear function.
forward(input)[source][source]

It’s also known as identity activation funtion. The forward step is $$\varphi(x) = x$$

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the identity applied to the activation.


class Linear(Activation):
"""Linear activation function.
"""

def __init__(self):
super(Linear, self).__init__()

def forward(self, input):
"""It's also known as identity activation funtion. The
forward step is :math:\\varphi(x) = x

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the identity applied to the activation.
"""
self.last_forward = input
return input

def derivative(self, input=None):
"""Backward propagation.
The backward also return identity matrix.

Returns
-------
float32
The derivative of linear function.
"""
last_forward = input if input else self.last_forward
return np.ones(last_forward.shape, dtype=get_dtype())


class npdl.activations.Softmax[source][source]

Softmax activation function.

derivative(input=None)[source][source]

Backward propagation.

Returns: float32 The derivative of Softmax function.
forward(input)[source][source]

$$\varphi(\mathbf{x})_j = \frac{e^{\mathbf{x}_j}}{\sum_{k=1}^K e^{\mathbf{x}_k}}$$ where $$K$$ is the total number of neurons in the layer. This activation function gets applied row-wise.

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 where the sum of the row is 1 and each single value is in [0, 1] The output of the softmax function applied to the activation.


class Softmax(Activation):
"""Softmax activation function.
"""

def __init__(self):
super(Softmax, self).__init__()

def forward(self, input):
""":math:\\varphi(\\mathbf{x})_j =
\\frac{e^{\mathbf{x}_j}}{\sum_{k=1}^K e^{\mathbf{x}_k}}
where :math:K is the total number of neurons in the layer. This
activation function gets applied row-wise.

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32 where the sum of the row is 1 and each single value is in [0, 1]
The output of the softmax function applied to the activation.
"""
assert np.ndim(input) == 2
self.last_forward = input
x = input - np.max(input, axis=1, keepdims=True)
exp_x = np.exp(x)
s = exp_x / np.sum(exp_x, axis=1, keepdims=True)
return s

def derivative(self, input=None):
"""Backward propagation.

Returns
-------
float32
The derivative of Softmax function.
"""
last_forward = input if input else self.last_forward
return np.ones(last_forward.shape, dtype=get_dtype())


class npdl.activations.Elliot(steepness=1)[source][source]

A fast approximation of sigmoid.

The function was first introduced in 1993 by D.L. Elliot under the title A Better Activation Function for Artificial Neural Networks. The function closely approximates the Sigmoid or Hyperbolic Tangent functions for small values, however it takes longer to converge for large values (i.e. It doesn’t go to 1 or 0 as fast), though this isn’t particularly a problem if you’re using it for classification.

derivative(input=None)[source][source]

Backward propagation.

Returns: float32 The derivative of Elliot function.
forward(input)[source][source]

Forward propagation.

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the softplus function applied to the activation.


class Elliot(Activation):
""" A fast approximation of sigmoid.

The function was first introduced
in 1993 by D.L. Elliot under the title A Better Activation Function for
Artificial Neural Networks. The function closely approximates the
Sigmoid or Hyperbolic Tangent functions for small values, however it
takes longer to converge for large values (i.e. It doesn't go to 1 or
0 as fast), though this isn't particularly a problem if you're using
it for classification.

"""

def __init__(self, steepness=1):
super(Elliot, self).__init__()

self.steepness = steepness

def forward(self, input):
"""Forward propagation.

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the softplus function applied to the activation.
"""
self.last_forward = 1 + np.abs(input * self.steepness)
return 0.5 * self.steepness * input / self.last_forward + 0.5

def derivative(self, input=None):
"""Backward propagation.

Returns
-------
float32
The derivative of Elliot function.
"""
last_forward = 1 + np.abs(input * self.steepness) if input else self.last_forward
return 0.5 * self.steepness / np.power(last_forward, 2)


class npdl.activations.SymmetricElliot(steepness=1)[source][source]

Elliot symmetric sigmoid transfer function.

derivative(input=None)[source][source]

Backward propagation.

Returns: float32 The derivative of SymmetricElliot function.
forward(input)[source][source]

Forward propagation.

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the softplus function applied to the activation.


class SymmetricElliot(Activation):
"""Elliot symmetric sigmoid transfer function.

"""

def __init__(self, steepness=1):
super(SymmetricElliot, self).__init__()
self.steepness = steepness

def forward(self, input):
"""Forward propagation.

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the softplus function applied to the activation.
"""
self.last_forward = 1 + np.abs(input * self.steepness)
return input * self.steepness / self.last_forward

def derivative(self, input=None):
"""Backward propagation.

Returns
-------
float32
The derivative of SymmetricElliot function.
"""
last_forward = 1 + np.abs(input * self.steepness) if input else self.last_forward
return self.steepness / np.power(last_forward, 2)


class npdl.activations.SoftPlus[source][source]

Softplus activation function.

derivative(input=None)[source][source]

Backward propagation.

Returns: float32 The derivative of Softplus function.
forward(input)[source][source]

$$\varphi(x) = \log(1 + e^x)$$

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the softplus function applied to the activation.


class SoftPlus(Activation):
"""Softplus activation function.
"""

def __init__(self):
super(SoftPlus, self).__init__()

def forward(self, input):
""":math:\\varphi(x) = \\log(1 + e^x)

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the softplus function applied to the activation.
"""
self.last_forward = np.exp(input)
return np.log(1 + self.last_forward)

def derivative(self, input=None):
"""Backward propagation.

Returns
-------
float32
The derivative of Softplus function.
"""
last_forward = np.exp(input) if input else self.last_forward
return last_forward / (1 + last_forward)


class npdl.activations.SoftSign[source][source]

SoftSign activation function.

derivative(input=None)[source][source]

Backward propagation.

Returns: float32 The derivative of SoftSign function.
forward(input)[source][source]

Forward propagation.

Parameters: x : float32 The activation (the summed, weighted input of a neuron). float32 The output of the softplus function applied to the activation.


class SoftSign(Activation):
"""SoftSign activation function.

"""

def __init__(self):
super(SoftSign, self).__init__()

def forward(self, input):
"""Forward propagation.

Parameters
----------
x : float32
The activation (the summed, weighted input of a neuron).

Returns
-------
float32
The output of the softplus function applied to the activation.
"""
self.last_forward = np.abs(input) + 1
return input / self.last_forward

def derivative(self, input=None):
"""Backward propagation.

Returns
-------
float32
The derivative of SoftSign function.
"""
last_forward = np.abs(input) + 1 if input else self.last_forward
return 1. / np.power(last_forward, 2)