Activation Functions in Neural Networks
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Training Autoencoders by Alternating Minimization
Under review as a conference paper at ICLR 2018 TRAINING AUTOENCODERS BY ALTERNATING MINI- MIZATION Anonymous authors Paper under double-blind review ABSTRACT We present DANTE, a novel method for training neural networks, in particular autoencoders, using the alternating minimization principle. DANTE provides a distinct perspective in lieu of traditional gradient-based backpropagation techniques commonly used to train deep networks. It utilizes an adaptation of quasi-convex optimization techniques to cast autoencoder training as a bi-quasi-convex optimiza- tion problem. We show that for autoencoder configurations with both differentiable (e.g. sigmoid) and non-differentiable (e.g. ReLU) activation functions, we can perform the alternations very effectively. DANTE effortlessly extends to networks with multiple hidden layers and varying network configurations. In experiments on standard datasets, autoencoders trained using the proposed method were found to be very promising and competitive to traditional backpropagation techniques, both in terms of quality of solution, as well as training speed. 1 INTRODUCTION For much of the recent march of deep learning, gradient-based backpropagation methods, e.g. Stochastic Gradient Descent (SGD) and its variants, have been the mainstay of practitioners. The use of these methods, especially on vast amounts of data, has led to unprecedented progress in several areas of artificial intelligence. On one hand, the intense focus on these techniques has led to an intimate understanding of hardware requirements and code optimizations needed to execute these routines on large datasets in a scalable manner. Today, myriad off-the-shelf and highly optimized packages exist that can churn reasonably large datasets on GPU architectures with relatively mild human involvement and little bootstrap effort. -
Lecture 4 Feedforward Neural Networks, Backpropagation
CS7015 (Deep Learning): Lecture 4 Feedforward Neural Networks, Backpropagation Mitesh M. Khapra Department of Computer Science and Engineering Indian Institute of Technology Madras 1/9 Mitesh M. Khapra CS7015 (Deep Learning): Lecture 4 References/Acknowledgments See the excellent videos by Hugo Larochelle on Backpropagation 2/9 Mitesh M. Khapra CS7015 (Deep Learning): Lecture 4 Module 4.1: Feedforward Neural Networks (a.k.a. multilayered network of neurons) 3/9 Mitesh M. Khapra CS7015 (Deep Learning): Lecture 4 The input to the network is an n-dimensional hL =y ^ = f(x) vector The network contains L − 1 hidden layers (2, in a3 this case) having n neurons each W3 b Finally, there is one output layer containing k h 3 2 neurons (say, corresponding to k classes) Each neuron in the hidden layer and output layer a2 can be split into two parts : pre-activation and W 2 b2 activation (ai and hi are vectors) h1 The input layer can be called the 0-th layer and the output layer can be called the (L)-th layer a1 W 2 n×n and b 2 n are the weight and bias W i R i R 1 b1 between layers i − 1 and i (0 < i < L) W 2 n×k and b 2 k are the weight and bias x1 x2 xn L R L R between the last hidden layer and the output layer (L = 3 in this case) 4/9 Mitesh M. Khapra CS7015 (Deep Learning): Lecture 4 hL =y ^ = f(x) The pre-activation at layer i is given by ai(x) = bi + Wihi−1(x) a3 W3 b3 The activation at layer i is given by h2 hi(x) = g(ai(x)) a2 W where g is called the activation function (for 2 b2 h1 example, logistic, tanh, linear, etc.) The activation at the output layer is given by a1 f(x) = h (x) = O(a (x)) W L L 1 b1 where O is the output activation function (for x1 x2 xn example, softmax, linear, etc.) To simplify notation we will refer to ai(x) as ai and hi(x) as hi 5/9 Mitesh M. -
Revisiting the Softmax Bellman Operator: New Benefits and New Perspective
Revisiting the Softmax Bellman Operator: New Benefits and New Perspective Zhao Song 1 * Ronald E. Parr 1 Lawrence Carin 1 Abstract tivates the use of exploratory and potentially sub-optimal actions during learning, and one commonly-used strategy The impact of softmax on the value function itself is to add randomness by replacing the max function with in reinforcement learning (RL) is often viewed as the softmax function, as in Boltzmann exploration (Sutton problematic because it leads to sub-optimal value & Barto, 1998). Furthermore, the softmax function is a (or Q) functions and interferes with the contrac- differentiable approximation to the max function, and hence tion properties of the Bellman operator. Surpris- can facilitate analysis (Reverdy & Leonard, 2016). ingly, despite these concerns, and independent of its effect on exploration, the softmax Bellman The beneficial properties of the softmax Bellman opera- operator when combined with Deep Q-learning, tor are in contrast to its potentially negative effect on the leads to Q-functions with superior policies in prac- accuracy of the resulting value or Q-functions. For exam- tice, even outperforming its double Q-learning ple, it has been demonstrated that the softmax Bellman counterpart. To better understand how and why operator is not a contraction, for certain temperature pa- this occurs, we revisit theoretical properties of the rameters (Littman, 1996, Page 205). Given this, one might softmax Bellman operator, and prove that (i) it expect that the convenient properties of the softmax Bell- converges to the standard Bellman operator expo- man operator would come at the expense of the accuracy nentially fast in the inverse temperature parameter, of the resulting value or Q-functions, or the quality of the and (ii) the distance of its Q function from the resulting policies. -
Improvements on Activation Functions in ANN: an Overview
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CSCanada.net: E-Journals (Canadian Academy of Oriental and Occidental Culture,... ISSN 1913-0341 [Print] Management Science and Engineering ISSN 1913-035X [Online] Vol. 14, No. 1, 2020, pp. 53-58 www.cscanada.net DOI:10.3968/11667 www.cscanada.org Improvements on Activation Functions in ANN: An Overview YANG Lexuan[a],* [a]Hangzhou No.2 High School of Zhejiang Province; Hangzhou, China. An ANN is comprised of computable units called * Corresponding author. neurons (or nodes). The activation function in a neuron processes the inputted signals and determines the output. Received 9 April 2020; accepted 21 May 2020 Published online 26 June 2020 Activation functions are particularly important in the ANN algorithm; change in activation functions can have a significant impact on network performance. Therefore, Abstract in recent years, researches have been done in depth on Activation functions are an essential part of artificial the improvement of activation functions to solve or neural networks. Over the years, researches have been alleviate some problems encountered in practices with done to seek for new functions that perform better. “classic” activation functions. This paper will give some There are several mainstream activation functions, such brief examples and discussions on the modifications and as sigmoid and ReLU, which are widely used across improvements of mainstream activation functions. the decades. At the meantime, many modified versions of these functions are also proposed by researchers in order to further improve the performance. In this paper, 1. INTRODUCTION OF ACTIVATION limitations of the mainstream activation functions, as well as the main characteristics and relative performances of FUNCTIONS their modifications, are discussed. -
CS 189 Introduction to Machine Learning Spring 2021 Jonathan Shewchuk HW6
CS 189 Introduction to Machine Learning Spring 2021 Jonathan Shewchuk HW6 Due: Wednesday, April 21 at 11:59 pm Deliverables: 1. Submit your predictions for the test sets to Kaggle as early as possible. Include your Kaggle scores in your write-up (see below). The Kaggle competition for this assignment can be found at • https://www.kaggle.com/c/spring21-cs189-hw6-cifar10 2. The written portion: • Submit a PDF of your homework, with an appendix listing all your code, to the Gradescope assignment titled “Homework 6 Write-Up”. Please see section 3.3 for an easy way to gather all your code for the submission (you are not required to use it, but we would strongly recommend using it). • In addition, please include, as your solutions to each coding problem, the specific subset of code relevant to that part of the problem. Whenever we say “include code”, that means you can either include a screenshot of your code, or typeset your code in your submission (using markdown or LATEX). • You may typeset your homework in LaTeX or Word (submit PDF format, not .doc/.docx format) or submit neatly handwritten and scanned solutions. Please start each question on a new page. • If there are graphs, include those graphs in the correct sections. Do not put them in an appendix. We need each solution to be self-contained on pages of its own. • In your write-up, please state with whom you worked on the homework. • In your write-up, please copy the following statement and sign your signature next to it. -
Can Temporal-Difference and Q-Learning Learn Representation? a Mean-Field Analysis
Can Temporal-Difference and Q-Learning Learn Representation? A Mean-Field Analysis Yufeng Zhang Qi Cai Northwestern University Northwestern University Evanston, IL 60208 Evanston, IL 60208 [email protected] [email protected] Zhuoran Yang Yongxin Chen Zhaoran Wang Princeton University Georgia Institute of Technology Northwestern University Princeton, NJ 08544 Atlanta, GA 30332 Evanston, IL 60208 [email protected] [email protected] [email protected] Abstract Temporal-difference and Q-learning play a key role in deep reinforcement learning, where they are empowered by expressive nonlinear function approximators such as neural networks. At the core of their empirical successes is the learned feature representation, which embeds rich observations, e.g., images and texts, into the latent space that encodes semantic structures. Meanwhile, the evolution of such a feature representation is crucial to the convergence of temporal-difference and Q-learning. In particular, temporal-difference learning converges when the function approxi- mator is linear in a feature representation, which is fixed throughout learning, and possibly diverges otherwise. We aim to answer the following questions: When the function approximator is a neural network, how does the associated feature representation evolve? If it converges, does it converge to the optimal one? We prove that, utilizing an overparameterized two-layer neural network, temporal- difference and Q-learning globally minimize the mean-squared projected Bellman error at a sublinear rate. Moreover, the associated feature representation converges to the optimal one, generalizing the previous analysis of [21] in the neural tan- gent kernel regime, where the associated feature representation stabilizes at the initial one. The key to our analysis is a mean-field perspective, which connects the evolution of a finite-dimensional parameter to its limiting counterpart over an infinite-dimensional Wasserstein space. -
On the Learning Property of Logistic and Softmax Losses for Deep Neural Networks
The Thirty-Fourth AAAI Conference on Artificial Intelligence (AAAI-20) On the Learning Property of Logistic and Softmax Losses for Deep Neural Networks Xiangrui Li, Xin Li, Deng Pan, Dongxiao Zhu∗ Department of Computer Science Wayne State University {xiangruili, xinlee, pan.deng, dzhu}@wayne.edu Abstract (unweighted) loss, resulting in performance degradation Deep convolutional neural networks (CNNs) trained with lo- for minority classes. To remedy this issue, the class-wise gistic and softmax losses have made significant advancement reweighted loss is often used to emphasize the minority in visual recognition tasks in computer vision. When training classes that can boost the predictive performance without data exhibit class imbalances, the class-wise reweighted ver- introducing much additional difficulty in model training sion of logistic and softmax losses are often used to boost per- (Cui et al. 2019; Huang et al. 2016; Mahajan et al. 2018; formance of the unweighted version. In this paper, motivated Wang, Ramanan, and Hebert 2017). A typical choice of to explain the reweighting mechanism, we explicate the learn- weights for each class is the inverse-class frequency. ing property of those two loss functions by analyzing the nec- essary condition (e.g., gradient equals to zero) after training A natural question then to ask is what roles are those CNNs to converge to a local minimum. The analysis imme- class-wise weights playing in CNN training using LGL diately provides us explanations for understanding (1) quan- or SML that lead to performance gain? Intuitively, those titative effects of the class-wise reweighting mechanism: de- weights make tradeoffs on the predictive performance terministic effectiveness for binary classification using logis- among different classes. -
CS281B/Stat241b. Statistical Learning Theory. Lecture 7. Peter Bartlett
CS281B/Stat241B. Statistical Learning Theory. Lecture 7. Peter Bartlett Review: ERM and uniform laws of large numbers • 1. Rademacher complexity 2. Tools for bounding Rademacher complexity Growth function, VC-dimension, Sauer’s Lemma − Structural results − Neural network examples: linear threshold units • Other nonlinearities? • Geometric methods • 1 ERM and uniform laws of large numbers Empirical risk minimization: Choose fn F to minimize Rˆ(f). ∈ How does R(fn) behave? ∗ For f = arg minf∈F R(f), ∗ ∗ ∗ ∗ R(fn) R(f )= R(fn) Rˆ(fn) + Rˆ(fn) Rˆ(f ) + Rˆ(f ) R(f ) − − − − ∗ ULLN for F ≤ 0 for ERM LLN for f |sup R{z(f) Rˆ}(f)| + O(1{z/√n).} | {z } ≤ f∈F − 2 Uniform laws and Rademacher complexity Definition: The Rademacher complexity of F is E Rn F , k k where the empirical process Rn is defined as n 1 R (f)= ǫ f(X ), n n i i i=1 X and the ǫ1,...,ǫn are Rademacher random variables: i.i.d. uni- form on 1 . {± } 3 Uniform laws and Rademacher complexity Theorem: For any F [0, 1]X , ⊂ 1 E Rn F O 1/n E P Pn F 2E Rn F , 2 k k − ≤ k − k ≤ k k p and, with probability at least 1 2exp( 2ǫ2n), − − E P Pn F ǫ P Pn F E P Pn F + ǫ. k − k − ≤ k − k ≤ k − k Thus, P Pn F E Rn F , and k − k ≈ k k R(fn) inf R(f)= O (E Rn F ) . − f∈F k k 4 Tools for controlling Rademacher complexity 1. -
Neural Networks Shuai Li John Hopcroft Center, Shanghai Jiao Tong University
Lecture 6: Neural Networks Shuai Li John Hopcroft Center, Shanghai Jiao Tong University https://shuaili8.github.io https://shuaili8.github.io/Teaching/VE445/index.html 1 Outline • Perceptron • Activation functions • Multilayer perceptron networks • Training: backpropagation • Examples • Overfitting • Applications 2 Brief history of artificial neural nets • The First wave • 1943 McCulloch and Pitts proposed the McCulloch-Pitts neuron model • 1958 Rosenblatt introduced the simple single layer networks now called Perceptrons • 1969 Minsky and Papert’s book Perceptrons demonstrated the limitation of single layer perceptrons, and almost the whole field went into hibernation • The Second wave • 1986 The Back-Propagation learning algorithm for Multi-Layer Perceptrons was rediscovered and the whole field took off again • The Third wave • 2006 Deep (neural networks) Learning gains popularity • 2012 made significant break-through in many applications 3 Biological neuron structure • The neuron receives signals from their dendrites, and send its own signal to the axon terminal 4 Biological neural communication • Electrical potential across cell membrane exhibits spikes called action potentials • Spike originates in cell body, travels down axon, and causes synaptic terminals to release neurotransmitters • Chemical diffuses across synapse to dendrites of other neurons • Neurotransmitters can be excitatory or inhibitory • If net input of neuro transmitters to a neuron from other neurons is excitatory and exceeds some threshold, it fires an action potential 5 Perceptron • Inspired by the biological neuron among humans and animals, researchers build a simple model called Perceptron • It receives signals 푥푖’s, multiplies them with different weights 푤푖, and outputs the sum of the weighted signals after an activation function, step function 6 Neuron vs. -
Loss Function Search for Face Recognition
Loss Function Search for Face Recognition Xiaobo Wang * 1 Shuo Wang * 1 Cheng Chi 2 Shifeng Zhang 2 Tao Mei 1 Abstract Generally, the CNNs are equipped with classification loss In face recognition, designing margin-based (e.g., functions (Liu et al., 2017; Wang et al., 2018f;e; 2019a; Yao angular, additive, additive angular margins) soft- et al., 2018; 2017; Guo et al., 2020), metric learning loss max loss functions plays an important role in functions (Sun et al., 2014; Schroff et al., 2015) or both learning discriminative features. However, these (Sun et al., 2015; Wen et al., 2016; Zheng et al., 2018b). hand-crafted heuristic methods are sub-optimal Metric learning loss functions such as contrastive loss (Sun because they require much effort to explore the et al., 2014) or triplet loss (Schroff et al., 2015) usually large design space. Recently, an AutoML for loss suffer from high computational cost. To avoid this problem, function search method AM-LFS has been de- they require well-designed sample mining strategies. So rived, which leverages reinforcement learning to the performance is very sensitive to these strategies. In- search loss functions during the training process. creasingly more researchers shift their attention to construct But its search space is complex and unstable that deep face recognition models by re-designing the classical hindering its superiority. In this paper, we first an- classification loss functions. alyze that the key to enhance the feature discrim- Intuitively, face features are discriminative if their intra- ination is actually how to reduce the softmax class compactness and inter-class separability are well max- probability. -
Deep Neural Networks for Choice Analysis: Architecture Design with Alternative-Specific Utility Functions Shenhao Wang Baichuan
Deep Neural Networks for Choice Analysis: Architecture Design with Alternative-Specific Utility Functions Shenhao Wang Baichuan Mo Jinhua Zhao Massachusetts Institute of Technology Abstract Whereas deep neural network (DNN) is increasingly applied to choice analysis, it is challenging to reconcile domain-specific behavioral knowledge with generic-purpose DNN, to improve DNN's interpretability and predictive power, and to identify effective regularization methods for specific tasks. To address these challenges, this study demonstrates the use of behavioral knowledge for designing a particular DNN architecture with alternative-specific utility functions (ASU-DNN) and thereby improving both the predictive power and interpretability. Unlike a fully connected DNN (F-DNN), which computes the utility value of an alternative k by using the attributes of all the alternatives, ASU-DNN computes it by using only k's own attributes. Theoretically, ASU- DNN can substantially reduce the estimation error of F-DNN because of its lighter architecture and sparser connectivity, although the constraint of alternative-specific utility can cause ASU- DNN to exhibit a larger approximation error. Empirically, ASU-DNN has 2-3% higher prediction accuracy than F-DNN over the whole hyperparameter space in a private dataset collected in Singapore and a public dataset available in the R mlogit package. The alternative-specific connectivity is associated with the independence of irrelevant alternative (IIA) constraint, which as a domain-knowledge-based regularization method is more effective than the most popular generic-purpose explicit and implicit regularization methods and architectural hyperparameters. ASU-DNN provides a more regular substitution pattern of travel mode choices than F-DNN does, rendering ASU-DNN more interpretable. -
Pseudo-Learning Effects in Reinforcement Learning Model-Based Analysis: a Problem Of
Pseudo-learning effects in reinforcement learning model-based analysis: A problem of misspecification of initial preference Kentaro Katahira1,2*, Yu Bai2,3, Takashi Nakao4 Institutional affiliation: 1 Department of Psychology, Graduate School of Informatics, Nagoya University, Nagoya, Aichi, Japan 2 Department of Psychology, Graduate School of Environment, Nagoya University 3 Faculty of literature and law, Communication University of China 4 Department of Psychology, Graduate School of Education, Hiroshima University 1 Abstract In this study, we investigate a methodological problem of reinforcement-learning (RL) model- based analysis of choice behavior. We show that misspecification of the initial preference of subjects can significantly affect the parameter estimates, model selection, and conclusions of an analysis. This problem can be considered to be an extension of the methodological flaw in the free-choice paradigm (FCP), which has been controversial in studies of decision making. To illustrate the problem, we conducted simulations of a hypothetical reward-based choice experiment. The simulation shows that the RL model-based analysis reports an apparent preference change if hypothetical subjects prefer one option from the beginning, even when they do not change their preferences (i.e., via learning). We discuss possible solutions for this problem. Keywords: reinforcement learning, model-based analysis, statistical artifact, decision-making, preference 2 Introduction Reinforcement-learning (RL) model-based trial-by-trial analysis is an important tool for analyzing data from decision-making experiments that involve learning (Corrado and Doya, 2007; Daw, 2011; O’Doherty, Hampton, and Kim, 2007). One purpose of this type of analysis is to estimate latent variables (e.g., action values and reward prediction error) that underlie computational processes.