CICLAD: A Fast and Memory-efficient Closed Itemset Miner for Streams

Mining association rules from data streams is a challenging task due to the (typically) limited resources available vs. the large size of the result. Frequent closed itemsets (FCI) enable an efficient first step, yet current FCI stream miners are not optimal on resource consumption, e.g. they store a large number of extra itemsets at an additional cost. In a search for a better storage-efficiency trade-off, we designed Ciclad,an intersection-based sliding-window FCI miner. Leveraging in-depth insights into FCI evolution, it combines minimal storage with quick access. Experimental results indicate Ciclad's memory imprint is much lower and its performances globally better than competitor methods.Comment: KDD2

Underdetermined-order recursive least-squares adaptive filtering: The concept and algorithms

Published versio

Embedding Stacked Polytopes on a Polynomial-Size Grid

A stacking operation adds a $d$-simplex on top of a facet of a simplicial $d$-polytope while maintaining the convexity of the polytope. A stacked $d$-polytope is a polytope that is obtained from a $d$-simplex and a series of stacking operations. We show that for a fixed $d$ every stacked $d$-polytope with $n$ vertices can be realized with nonnegative integer coordinates. The coordinates are bounded by $O(n^{2\log(2d)})$, except for one axis, where the coordinates are bounded by $O(n^{3\log(2d)})$. The described realization can be computed with an easy algorithm. The realization of the polytopes is obtained with a lifting technique which produces an embedding on a large grid. We establish a rounding scheme that places the vertices on a sparser grid, while maintaining the convexity of the embedding.Comment: 22 pages, 10 Figure

엔트로피 제어 비정질 형성 합금 시스템에서 비정질 형성능 및 기계적 특성 변화 고찰

학위논문(석사)--서울대학교 대학원 :공과대학 재료공학부,2019. 8. 박은수.Herein, we systematically investigated the effect of configuration entropy (CE) on the glass-forming ability (GFA) and the mechanical response in a series of equiatomic binary Cu50Zr50 to denary (CuNiBeCoFe)50(ZrTiHfTaNb)50 metallic glasses (MGs) with similar atomic size difference and heat of mixing through alloy design involving careful selection of elements chemically and topologically similar to Cu and Zr and subsequent substitution of pre-constituent elements. Interestingly, the senary (CuNiBe)50(ZrTiHf)50 MG with relatively medium CE value of 1.79R exhibited the maximum GFA among the investigated MGs, implying that the CE is not the dominant factor for GFA. The mechanical response analysis was comprehensively performed using nanoindentation test including deformation dynamics of shear avalanche through statistical analysis of pop-in behavior and the analysis result was compared to the atomic-level structure data obtained by high energy X-ray scattering experiment. The overall trend of the nanohardness and the Youngs modulus (E) was shown to outwardly increase which is dominantly due to the increased 3 atom-connection of polyhedra as well as lower fragility. However, the severe local structural irregularity and compositional complexity in MG with higher CE facilitate the chaotic deformation behavior that results in the unanticipated local softening of amorphous phase and ultimately modulate the response towards ductile deformation. Consequently, it can be concluded that the CE could be one of the crucial factors in designing an MG to alter its characteristics towards achieving desirable properties such as optimized GFA and enhanced ductility.본 연구에서는 구성 엔트로피가 비정질 형성능 및 기계적 거동에 미치는 영향을 알아보기 위해 이성분계 Cu50Zr50 부터 십성분계 (CuNiBeCoFe)50(ZrTiHfTaNb)50에 이르기 까지 다양한 금속원소들이 등가원소비로 이루어진 비정질 합금 시리즈의 기계적, 구조적 특성에 대해 분석하였다. 첨가 원소들간 원자 반경 차이, 혼합열 등의 특성이 비슷해 구성 엔트로피 효과를 확인하기 적합한 등가원소비 비정질 합금 시스템을 제조하기 위해 Cu 및 Zr과 화학적, 구조적 특성이 유사한 금속들을 선별하여 등가원소비로 첨가한 뒤 비정질 합금을 제조하였다. 특이하게도 비교적으로 중간 구성 엔트로피를 값을 (1.79R) 갖는 육성분계 (CuNiBe)50(ZrTiHf)50 비정질 합금이 가장 높은 비정질 형성능을 보이며 구성 엔트로피가 비정질 형성능의 우성인자가 아닌 것을 알 수 있었다. 구성원소 개수에 따른 구성 엔트로피 변화가 기계적 거동에 미치는 영향을 확인하기 위해 나노압입 시험을 통한 pop-in 거동의 통계적 분석을 통해 수행하였으며, 이를 이해하기 위해 고 에너지 X-선 산란 실험 (high energy X-ray scattering) 에 의해 얻어진 원자 단위 구조 데이터와 비교 하여 논하였다. 그 결과, 원자 클러스터 간 결합 패턴의 비율이 면결합 (3 atom-connection)으로 증가 및 fragility는 감소할수록 합금의 경도 및 탄성계수 변화는 전반적으로 증가함을 확인할 수 있었다. 또한 원소 첨가에 따른 구성 엔트로피 증가는 국부적으로 구조적 불규칙성 및 조성적 복잡성을 유발 하여 chaotic 변형 거동을 촉진시켜 국부적 softening 현상이 나타나게 하였으며 궁극적으로 연성 변형에 대한 반응을 조절 시킬 수 있었다. 결과적으로, 구성 엔트로피는 최적의 비정질 형성능 및 향상된 연성과 같은 우수한 특성을 달성하기 위해 비정질 합금 설계에 중요한 요소임을 알 수 있었다.Chapter 1. Introduction 1 1.1. Metallic glass 1 1.1.1. Definition and characteristics 1 1.1.2. Glass-forming ability parameter 4 1.1.3. Fragility 9 1.1.4. Deformation mechanism 11 1.1.5. Shear avalanche in metallic glass 13 1.2. High entropy alloy 16 1.2.1. Definition and characteristics 16 1.2.2. Four core effects of HEA 19 1.3. High entropy metallic glass 22 1.3.1. Properties and the current understanding 23 1.4. Thesis objective and research strategy 27 Chapter 2. Experimental procedure 29 2.1. Sample preparation 29 2.1.1. Fabrication of metallic glass ribbon 29 2.2. Structural analysis 30 2.2.1. X-ray diffraction 30 2.2.2. Transmission electron microscopy 30 2.2.3. High energy X-ray scattering 30 2.3. Thermal analysis 34 2.3.1. Differential scanning calorimetry 34 2.4. Mechanical testing 35 2.4.1. Nanoindentation test 35 Chapter 3. Results 37 3.1. Alloy design 37 3.2. Structural analysis 42 3.2.1. X-ray diffraction analysis 42 3.2.2. Transmission electron microscopy analysis 42 3.2.3. High energy X-ray scattering analysis 46 3.3. Glass-forming ability evaluation 48 3.4. Fragility evaluation 53 3.5. Nanoindentation test 55 Chapter 4. Discussion 60 4.1. Influence of configuration entropy on the atomic-level structure 60 4.2. Anomalous modulus variation in high entropy metallic glass 65 4.3. Anomalous deformation dynamics of high entropy metallic glass 70 Chapter 5. Conclusion 78Maste

Achieving K-Anonymity Using Parallelism in Full Domain Generalization

Preserving privacy while publishing data has emerged as key research area in data security and has become a primary issue in publishing person specific sensitive information. How to preserve one’s privacy efficiently is a critical issue while publishing data. k - Anonymity is a key technique for de-identifying the sensitive datasets. In our work, we have described an approach to implement various k anonymity algorithms and also propose a parallelism method that produces better results with the real-world datasets. Additionally, we suggest a new approach that attains better results by applying a parallelism approach and exploiting various characteristics of our suggested approach. The proposed approach uses the concept of samarati algorithm to generalize the lattice and uses the binary search method. The proposed algorithm generates the levels using binary search in the lattice and then uses the parallel mechanism for evaluating the nodes. The proposed algorithm has less execution time than other full domain generalization algorithms for k -anonymization

Invariant preservation in geo-replicated data stores

The Internet has enabled people from all around the globe to communicate with each other in a matter of milliseconds. This possibility has a great impact in the way we work, behave and communicate, while the full extent of possibilities are yet to be known. As we become more dependent of Internet services, the more important is to ensure that these systems operate correctly, with low latency and high availability for millions of clients scattered all around the globe. To be able to provide service to a large number of clients, and low access latency for clients in different geographical locations, Internet services typically rely on georeplicated storage systems. Replication comes with costs that may affect service quality. To propagate updates between replicas, systems either choose to lose consistency in favor of better availability and latency (weak consistency), or maintain consistency, but the system might become unavailable during partitioning (strong consistency). In practice, many production systems rely on weak consistency storage systems to enhance user experience, overlooking that applications can become incorrect due to the weaker consistency assumptions. In this thesis, we study how to exploit application’s semantics to build correct applications without affecting the availability and latency of operations. We propose a new consistency model that breaks apart from traditional knowledge that applications consistency is dependent on coordinating the execution of operations across replicas. We show that it is possible to execute most operations with low latency and in an highly available way, while preserving application’s correctness. Our approach consists in specifying the fundamental properties that define the correctness of applications, i.e. the application invariants, and identify and prevent concurrent executions that potentially can make the state of the database inconsistent, i.e. that may violate some invariant. We explore different, complementary, approaches to implement this model. The Indigo approach consists in preventing conflicting operations from executing concurrently, by restricting the operations that each replica can execute at each moment to maintain application’s correctness. The IPA approach does not preclude the execution of any operation, ensuring high availability. To maintain application correctness, operations are modified to prevent invariant violations during replica reconciliation, or, if modifying operations provides an unsatisfactory semantics, it is possible to correct any invariant violations before a client can read an inconsistent state, by executing compensations. Evaluation shows that our approaches can ensure both low latency and high availability for most operations in common Internet application workloads, with small execution overhead in comparison to unmodified weak consistency systems, while enforcing application invariants, as in strong consistency systems