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Create a Sequential model

We will create a Sequential network with four layers.

Layer 1 is a dense layer which has input_shape of (*, 784) and an output_shape of (*, 32)

A dense layer is a regular densely-connected neural network layer. A Dense layer implements the operation output = activation(dot(input, kernel) + bias), where activation is the element-wise activation function passed as the activation argument, kernel is a weights matrix created by the layer, and bias is a bias vector created by the layer. (This is only applicable if use_bias is True).

Layer 2 is an activation layer with the tanh Activation function applies activation to the incoming tensor:

keras.layers.Activation(activation)

Activation can also be applied as a parameter to the dense layer:

model.add(Dense(64, activation='tanh'))

Layer 3 is a dense layer with output (*,10)
Layer 4 has Activation that applies the softmax function:

Activation('softmax')

In mathematics, the softmax function, also called the normalized exponential function, is a generalization of the logistic function that squashes a K-dimensional vector z of arbitrary real values to a K-dimensional vector σ(z) of real values; each entry is in the range (0, 1), and all the entries add up to 1. The following formula shows this:

from keras.models import Sequential
from keras.layers import Dense, Activation
model = Sequential([
  Dense(32, input_shape=(784,)),
  Activation('tanh'),
  Dense(10),
  Activation('softmax'),
])
print(model.summary())

The summary of the model created is printed in the following snippet:

Layer (type) Output Shape Param # 
=================================================================
dense_1 (Dense) (None, 32) 25120 
_________________________________________________________________
activation_1 (Activation) (None, 32) 0 
_________________________________________________________________
dense_2 (Dense) (None, 10) 330 
_________________________________________________________________
activation_2 (Activation) (None, 10) 0 
=================================================================
Total params: 25,450
Trainable params: 25,450
Non-trainable params: 0
_______________________________________________________________

Sequential is a subclass of Model and has some additional methods, as shown in the following sections.