NOMURA, Ryo

写真a

Affiliation

Affiliated organization, Center for Data Science

Job title

Professor

Concurrent Post 【 display / non-display

  • Affiliated organization   Global Education Center

Education 【 display / non-display

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    Waseda University   School of Science and Engineering  

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    Waseda University  

Degree 【 display / non-display

  • 早稲田大学   博士(工学)

Research Experience 【 display / non-display

  • 2019.04
    -
    Now

    Waseda University   Center for Data Science   Professor

  • 2018.04
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    2019.03

    Senshu University   School of Network and Information   Professor

  • 2012.04
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    2018.03

    Senshu University   School of Network and Information   Associate Professor

  • 2010.04
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    2012.03

    Senshu University   School of Network and Information   Lecturer

  • 2008.04
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    2010.03

    Assistant Professor

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Professional Memberships 【 display / non-display

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    IEEE

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    電子情報通信学会

 

Research Areas 【 display / non-display

  • Theory of informatics   Theory of informatics

  • Communication and network engineering   Communication/Network engineering

Research Interests 【 display / non-display

  • >Information Theory

  • >Information Theory, Shannon Theory

Papers 【 display / non-display

  • Source Resolvability and Intrinsic Randomness: Two Random Number Generation Problems With Respect to a Subclass of f-Divergences

    Ryo Nomura

    IEEE TRANSACTIONS ON INFORMATION THEORY   66 ( 12 ) 7588 - 7601  2020.12

     View Summary

    This paper deals with two typical random number generation problems in information theory. One is the source resolvability problem (resolvability problem for short) and the other is the intrinsic randomness problem. In the literature, optimum achievable rates in these two problems with respect to the variational distance as well as the Kullback-Leibler (KL) divergence have already been analyzed. On the other hand, in this study we consider these two problems with respect to f-divergences. The f-divergence is a general non-negative measure between two probabilistic distributions on the basis of a convex function f. The class of f-divergences includes several important measures such as the variational distance, the KL divergence, the Hellinger distance and so on. Hence, it is meaningful to consider the random number generation problems with respect to f-divergences. In this paper, we impose some conditions on the function f so as to simplify the analysis, that is, we consider a subclass of f-divergences. Then, we first derive general formulas of the first-order optimum achievable rates with respect to f-divergences. Next, we particularize our general formulas to several specified functions f. As a result, we reveal that it is easy to derive optimum achievable rates for several important measures from our general formulas. The second-order optimum achievable rates and optimistic optimum achievable rates have also been investigated.

    DOI

  • Stochastic Discrete First-Order Algorithm for Feature Subset Selection

    Kota Kudo, Yuichi Takano, Ryo Nomura

    IEICE TRANSACTIONS ON INFORMATION AND SYSTEMS   E103D ( 7 ) 1693 - 1702  2020.07

     View Summary

    This paper addresses the problem of selecting a significant subset of candidate features to use for multiple linear regression. Bertsimas et al. [5] recently proposed the discrete first-order (DFO) algorithm to efficiently find near-optimal solutions to this problem. However, this algorithm is unable to escape from locally optimal solutions. To resolve this, we propose a stochastic discrete first-order (SDFO) algorithm for feature subset selection. In this algorithm, random perturbations are added to a sequence of candidate solutions as a means to escape from locally optimal solutions, which broadens the range of discoverable solutions. Moreover, we derive the optimal step size in the gradient-descent direction to accelerate convergence of the algorithm. We also make effective use of the L-2-regularization term to improve the predictive performance of a resultant subset regression model. The simulation results demonstrate that our algorithm substantially outperforms the original DFO algorithm. Our algorithm was superior in predictive performance to lasso and forward stepwise selection as well.

    DOI

  • Overflow Probability of Variable-Length Codes With Codeword Cost

    Ryo Nomura

    IEEE TRANSACTIONS ON INFORMATION THEORY   65 ( 12 ) 8194 - 8206  2019.12

     View Summary

    Lossless variable-length source coding with codeword cost is considered for general sources. The problem setting, where we impose on unequal costs on code symbols, is called the variable-length coding with codeword cost. In this problem, the infimum of average codeword cost have already been determined for general sources. On the other hand, the overflow probability, which is defined as the probability of codeword cost being above a threshold, have not been considered yet. In this paper, we first determine the infimum of achievable threshold in the first-order sense and the second-order sense for general sources with additive memoryless codeword cost. Then, we compute it for some special sources such as i.i.d. sources and mixed sources. A generalization of the codeword cost is also discussed.

    DOI

  • Optimum Overflow Thresholds in Variable-Length Source Coding Allowing Non-Vanishing Error Probability

    Ryo Nomura, Hideki Yagi

    IEEE TRANSACTIONS ON INFORMATION THEORY   65 ( 12 ) 8213 - 8221  2019.12

     View Summary

    The variable-length source coding problem allowing the error probability up to some constant is considered for general sources. In this problem, the optimum mean codeword length of variable-length codes has already been determined. On the other hand, in this paper, we focus on the overflow (or excess codeword length) probability instead of the mean codeword length. The infimum of overflow thresholds under the constraint that both of the error probability and the overflow probability are smaller than or equal to some constant is called the optimum overflow threshold. In this paper, we first derive finite-length upper and lower bounds on these probabilities so as to analyze the optimum overflow thresholds. Then, by using these bounds, we determine the general formula of the optimum overflow thresholds in both of the first-order and second-order forms. Next, we consider another expression of the derived general formula so as to reveal the relationship with the optimum coding rate in the fixed-length source coding problem. Finally, we apply the general formula derived in this paper to the case of stationary memoryless sources.

    DOI

  • Source resolvability problem with respect to a certain subclass of f-divergence

    Ryo Nomura

    2019 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY (ISIT)     2234 - 2238  2019

     View Summary

    This paper deals with the source resolvability problem which is one of typical random number generation problems. In the literatures, the achievable rate in the source resolvability problem with respect to the variational distance as well as the Kullback-Leibler (KL) divergence have been analyzed. On the other hand, in this study we consider the problem with respect to a subclass of f-divergence. The f-divergence is a general non-negative measure between two probabilistic distributions and includes several important measures such as the variational distance, the KL divergence, the Hellinger distance and so on. Hence, it is meaningful to consider the source resolvability problem with respect to the f-divergence. We derive the general formula of the optimum achievable rate for a certain subclass of the f-divergence. Then, we reveal that it is easy to derive previous results from our general formula.

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Books and Other Publications 【 display / non-display

  • 漸近正規性を用いた可変長無歪み情報源符号化の性能評価に関する研究

    野村 亮( Part: Sole author)

    早稲田大学  2007.12

     View Summary

    学位論文

Misc 【 display / non-display

  • 「無歪み情報源符号化におけるオーバーフロー確率について」

    電子情報通信学会論文誌(電子情報通信学会)   J90-A ( 4 ) 292 - 304  2007

  • On the epsilon-overflow probability of lossless codes

    Ryo Nomura, Toshiyasu Matsushima, Shigeichi Hirasawa

    2007 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY PROCEEDINGS, VOLS 1-7   E90-A ( 12 ) 441 - +  2007

     View Summary

    In this paper, we generalize the achievability of variable-length coding from two viewpoints. One is the definition of an overflow probability, and the other is the definition of an achievability. We define the overflow probability as the probability of codeword length, not per symbol, is larger than eta(n) and we introduce the epsilon-achievability of variable-length codes that implies an existence of a code for the source under the condition that the overflow probability is smaller than or equal to epsilon.
    Then we show that the epsilon-achievabitity of variable-length codes is essentially equivalent to the epsilon-achievability of fixed-length codes for general sources. Moreover we show the condition of epsilon-achievability for some restricted sources given epsilon.

    DOI

Research Projects 【 display / non-display

  • Information Spectrum Approach to Source Coding Problems with Unknown Parameters

    Grant-in-Aid for Scientific Research (C)

    Project Year :

    2018.04
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    2022.03
     

  • Explicit Analysis of Multi-Terminal Information Theory with Unknown Parameters

    Grant-in-Aid for Scientific Research (C)

    Project Year :

    2014.04
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    2018.03
     

    Nomura Ryo

     View Summary

    We have analyzed fundamental properties of information theory. In particular, we have considered mixed sources and mixed channels, and derived the fundamental limits on the reliable communication. The situation where we know the class of probabilistic model but we do not know its parameter, can be formulated by the mixed probabilistic models. Hence, our approach is meaningful from the view point of the practical situation.
    We have determined the optimal first- and second-order achievable rates or rate regions in several problems.

  • A Unified Analysis and Optimization of Information Security System with Probabilistic Components from Viewpoints of Convenience and Safety

    Grant-in-Aid for Scientific Research (C)

    Project Year :

    2013.04
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    2016.03
     

    MATSUSHIMA Toshiyasu, UKITA Yoshifumi, YOSHIDA Takahiro, NOMURA Ryo, SUKO Tota, HORII Shunsuke

     View Summary

    Information security problem with probabilistic components has been formulated by probabilistic models. Theoretical criteria for evaluation such as convenience and safety have been defined clearly and optimal attack or an authentication method has been derived theoretically. Theoretical safety bounds have been evaluated with respect to mathematical models with unified framework for each cipher or security system. A theoretical safety bound or optimality has been clarified with respect to a tradeoff between convenience and safety. New theoretical criteria have been derived for information security systems. Approximation algorithms with high performance for optimal attack or an authentication method have been constructed applying results of studies on problems in related fields such as learning or optimization theory that is formulated by probabilistic models equivalent to our study. Convenience or safety of information security systems has been simulated by applying these algorithms.

  • Reliable communication on the sensor network with unknown probabilistic structure

    Grant-in-Aid for Young Scientists (B)

    Project Year :

    2011
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    2013
     

    NOMURA Ryo

     View Summary

    We have analyzed fundamental properties of sensor network on the basis of the multi-terminal information theory. In particular, we have considered the mixed sources and mixed channels, and derived the fundamental limits on the reliable communication. This is because some situations in which we know the class of probabilistic model but we do not know its parameter, can be formulated by the mixed models. We have determined the optimal second-order achievable rates in the single source coding problem and the optimal achievable rate region in the multi-terminal source coding problem. Moreover, we have observed that the optimal achievable rates in some problems on channel coding can be determined by using the similar approach.

  • A Study on Unified Attack against Stream Ciphers based on Probabilistic Inference Algorithms

    Grant-in-Aid for Scientific Research (C)

    Project Year :

    2010
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    2012
     

    MATSUSHIMA Toshiyasu, UKITA Yoshihumi, NOMURA Ryo

     View Summary

    Attack against Stream Cipher reduces to attack against a pseudo random generator. In this study, attack against a pseudo-random number generator was classified based on the choice of target variables and its order and the choice of the local relationships used for attack. We proposed efficient and accurate attack algorithms based on probabilistic inference by studying the problem in unified framework. We revealed the class of pseudo-random number generators that we can attack against based on the acquired knowledge of probabilistic inference so far.

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Presentations 【 display / non-display

  • f-ダイバージェンスの部分クラスと情報源Resolvability

    情報理論研究会  (やまと会議室(奈良県))  電子情報通信学会

    Presentation date: 2018.07

  • Source resolvability with Kullback-Leibler divergence

    2018 IEEE International Symposium on Information Theory  (Vail, Colorado, USA)  IEEE Information Theory Society

    Presentation date: 2018.06

  • Overflow Probability of Variable-Length Codes Allowing Non-Vanishing Error Probability

    IEEE Information Theory Workshop 2017  (Kaohsiung, Taiwan)  IEEE

    Presentation date: 2017.11

  • First- and Second-Order Optimistic Coding Theorems on Variable-Length Codes Allowing Non-Vanishing Error Probability

    IEICE Technical Committee Conference on Information Theory  IEICE

    Presentation date: 2017.09

  • First- and second-order hypothesis testing for mixed memoryless sources with general mixture

    2017 IEEE International Symposium on Information Theory  IEEE

    Presentation date: 2017.06

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Specific Research 【 display / non-display

  • 確率構造が未知の乱数生成問題に対する情報スペクトル的アプローチ

    2020  

     View Summary

    本研究は情報理論で近年発展している情報スペクトル理論を様々な問題に適用することを目標としたものである.特に,2020年度は確率構造が未知の状況における乱数生成問題を対象とした.情報理論における代表的な乱数生成問題としては,任意の与えられた確率分布を一様乱数を用いて近似する問題(RESOLVABILITY問題)および,一様乱数から任意の確率分布を近似する問題(INTRINSIC RANDOMNESS問題)の二つがある.いずれの問題においても重要な指標は,確率分布の近似尺度と一様乱数のサイズである.確率分布の近似尺度としては従来変動距離やKullback-Leibler距離などがあるが本研究ではこれをより一般的なf-ダイバージェンスまで拡張し,その上で,目的とする確率分布と作成した確率分布の間のf-ダイバージェンスが一定値以下になるもとでの一様乱数サイズについて分析することが目標となる.2020年度はResolvability問題における最適一様乱数サイズについて,従来得られている表現と異なる数量で評価できることが分かった.また同様の考えをIntrinsic Randomness問題に対して適用可能であることも分かった.今後この研究を発展させて確率構造が未知のIntrinsic Randomness問題について調査する.

  • ユニバーサル予測問題に対する情報スペクトル的アプローチ

    2019  

     View Summary

    本研究は情報理論で近年発展している情報スペクトル理論をユニバーサル予測問題に適用することを目標としたものである.本年度はその前準備として任意の確率分布を一様乱数を用いて近似する問題を考えた.この問題は情報源resolvability問題と呼ばれている.情報源resolvability問題の近似尺度として従来変動距離やKullback-Leibler距離などがあるが本研究ではこれをより一般的なf-ダイバージェンスまで拡張し,その上で情報源resolvability問題と情報源符号化問題の関係について考察した.これらの成果は今後論文誌に投稿予定である.

  • データ伝送・保存における高圧縮無杢符号化に関する研究

    2002  

     View Summary

    本研究では無歪データ圧縮符号の性質に関して評価を行った.無歪データ圧縮符号は通常平均符号長を用いて評価されており,その評価基準の下では最適な符号が提案されている.平均符号長はあくまで平均を評価しているためひとつの情報系列を符号化した際にその符号長が短くなるという保証はない.その様な視点から近年オーバーフロー確率という評価基準が提案された.これは符号長がある値より長くなってしまう確率を評価したものである.本研究では,このオーバーフロー確率についてまずオーバーフロー確率が最小になる符号を提案し,それを用いてオーバーフロー確率が最小になる限界を評価した.また,平均符号長の意味で最適な符号のオーバーフロー確率を評価した.これらの成果は今後論文誌に投稿予定である.

 

Syllabus 【 display / non-display

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Committee Memberships 【 display / non-display

  • 2018.06
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    電子情報通信学会 和文論文誌A編集委員

  • 2018.06
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    電子情報通信学会 英文論文誌A編集委員

  • 2018.01
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    Secretary of IEEE Information Theory Society Japan Chapter

  • 2015.05
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    2017.05

    電子情報通信学会 基礎・境界ソサイエティ特別委員

  • 2015.06
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    電子情報通信学会 コミュニケーション委員会 委員

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Social Activities 【 display / non-display

  • 電子情報通信学会 基礎・境界ソサイエティ 情報理論とその応用サブソサイエティ委員

    電子情報通信学会 

  • 電子情報通信学会 英文論文誌A 小特集(情報理論とその応用)編集幹事

    電子情報通信学会 

  • 第35回情報理論とその応用シンポジウム プログラム委員会幹事

    電子情報通信学会 基礎・境界ソサイエティ 情報理論とその応用サブソサイエティ 

  • 電子情報通信学会 情報理論研究専門委員会幹事

  • 電子情報通信学会 英文論文誌A 小特集(情報理論とその応用)編集委員

    電子情報通信学会 

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