Neural Network, dependence among outputs?

Question

Is there a way to train a neural network in the following manner:

You have $n$ observations in the training set.

The neural net will start with random weights, and produce $n$ outputs. I want to then apply a function, $f$, on these $n$ outputs. $f$ will output a single real valued number $\alpha$. The goal of the neural net is to then maximize $\alpha$.

That is, I don't care what the neural net outputs for any single observation. I want it to maximize some property of the outputs as a whole (e.g. I want to maximize the variance of the outputs).

Are there neural networks like this, or other techniques for what I am looking for?

Any insight is appreciated!

Answer 1

The neural net will start with random weights, and produce $n$ outputs. I want to then apply a function, $f$, on these $n$ outputs. $f$ will output a single real valued number $\alpha$. The goal of the neural net is to then maximize $\alpha$.

This means that the training set is not used. The problem is just maximizing $f(x_1, ..., x_n)$ given $x_1, ..., x_n$. There exist a fair amount of literature on that topic, the choice of technique depending on $f$: Mathematical optimization.

Answer 2

Yes, you can do it but with some modification to your proposed algorithm. In order to train your network, you need to know error (which you are computing), but also the direction of the error (derivative) with respect to the activity of the neuron and the weight. So you need to know if the error would increase or decrease if you change the weight.

So you can do

1. compute outputs for all your samples (and save activations of the neurons for each sample)
2. compute where should each of your sample move, to minimize your custom criterion.
3. us this as the error term in your training and update your weights accordingly

Another question is however if the neural network is the right tool to do it. For example if you want to find projection that would maximize variance, (nonlinear) PCA might be better.

Edit: Although after bounty period, but here is an example hacky code that does that.

This is a neural network trained to create outputs that are equally distant from each other.

import numpy as np
from sklearn import neural_network

def get_new_targets(x):
    """
    Function to get points that are in the middle of the neighbouring points"""
    try:
        x = [e[1] for e in x]
    except:
        pass
    enumerated_x = [e for e in enumerate(x)]
    enumerated_x.sort(key=lambda x: x[1])
    targets = []
    for i, elem in enumerate(enumerated_x):
        previous_sample, next_sample = 0, 0
#        current_sample =  enumerated_x[i][1]
        if i == 0:
            previous_sample = 0
        else:
            previous_sample = enumerated_x[i-1][1]
        if i == len(enumerated_x)-1:
            next_sample = 50
        else:            
            next_sample = enumerated_x[i+1][1]            
        target = (previous_sample + next_sample)/2. 
        targets.append(target)        
    targets_in_order = sorted([(enum[0], t) for enum, t in zip(enumerated_x, targets)])
    targets_in_order = [e[1] for e in targets_in_order]
    return np.array(targets_in_order)



# create training data
# in this example we care only about one axis
X = np.array([[0,3], [0,4], [0,10], [0,15], [0,35], [0,36] ,[0,50]])
# create initial targers. This will change every epoch
Y = get_new_targets(X)

preds = [] # save predictions
epochs = []

# warm start True and max_iter 1 to simulate batch training
clf = neural_network.MLPRegressor(max_iter=1, warm_start=True, alpha=0.005, 
                                  hidden_layer_sizes=(50,), activation='logistic')

for i in range(4000): # for each epoch
    # since warms_start=True it will reuse weights from previous epoch
    clf.fit(X, Y) 
    predictions =  clf.predict(X) 
    Y = get_new_targets(predictions)
    if i % 100 == 0:
        pred = clf.predict(X)        
        preds.append([pred])
        epochs.append(i)       

Plots that show that it kindof worked, but not really