Error Using sigmoid Function

import numpy as np

default_network():
    network={}
    network['W1'] = np.array([0.1, 0.3, 0.5], [0.2, 0.4, 0.6]])
    network ['b1'] = np.array ([0.1, 0.2, 0.3])
    network['W2'] = np.array([0.1, 0.4], [0.2, 0.5], [0.3, 0.6]])
    network['b2'] = np.array([0.1,0.2])
    network ['W3'] = np.array([0.1,0.3], [0.2,0.4]])
    network['b3'] = np.array([0.1,0.2])
    
    return network

def forward (network, x):
    W1, W2, W3 = network ['W1'], network ['W2', network ['W3']
    b1, b2, b3 = network['b1'], network['b2'], network['b3']
    
    a1 = np.dot(x,W1) + b1
    z1 = sigmoid(a1)
    a2 = np.dot(z1,W2) + b2
    z2 = sigmoid(a2)
    a3 = np.dot(z2,W3) + b3
    y = identity_function(a3)
    
    returny

network=init_network()
x = np.array ([1.0, 0.5])
y = forward (network, x)
print(y)

NameError Traceback (most recent call last)
<ipython-input-61-d459db580be1>in<module>
     27 network = init_network()
     28 x = np.array ([1.0, 0.5])
--- > 29y = forward (network, x)
     30 print(y)

<ipython-input-61-d459db580be1>in forward (network, x)
     17 
     18 a1 = np.dot(x, W1) + b1
--- > 19z1 = sigmoid(a1)
     20a2 = np.dot(z1,W2) + b2
     21 z2 = sigmoid(a2)

NameError: name 'sigmoid' is not defined