Judul: Penggerombolan Data Panel Emiten Sektor Pertambangan selama Pandemi Covid-19
Abstrak: The Covid-19 pandemic has made people start looking for new income, one of which is stock investment. Mining Stock recorded the highest sectoral index increase in 2020. The high increase in the mining sector index doesn’t indicate all of the stocks have a good performance. Clustering data of mining stock can help to see which stock has the best performance. Variables used in clustering are technical factors with details: return, trading volume, transaction frequency, bid volume, and foreign buy. Data in this research is longitudinal data from March 2020 until January 2022 and the clustering technique used is k-means. Clustering on outliers data and non-outliers data is done separately. Definition of outliers is exploratively with biplot analysis. Clustering on outliers data results obtained are five clusters and clustering on non-outliers data results obtained are two clusters. Best cluster is cluster who obtained ANTM because has highest value in return, transaction frequency, and foreign buy.
Keyword: clustering on longitudinal data, k-means, mining, stock, technical factor

Judul: Pengelompokan Emiten Berdasarkan Fluktuasi Harga Saham Syariah Menggunakan Algoritma Agglomerative Hierarchical Clustering
Abstrak: Kebijakan pemerintah untuk memutus rantai penularan Covid-19 memberikan dampak pada sektor ekonomi di Indonesia. Banyak pekerja yang harus mencari sumber pendapatan lain karena kehilangan pekerjaannya di masa pandemi. Salah satu usaha yang dilakukannya yaitu berinvestasi saham. Indonesia sebagai negara dengan mayoritas penduduk muslim membuat pasar modal syariah khususnya saham syariah semakin diminati. Namun demikian, saham syariah yang memiliki tingkat pengembalian tinggi juga memiliki risiko yang tinggi. Salah satu faktor yang membuat risiko saham tinggi adalah harga saham yang bersifat fluktuatif dan stokastik. Oleh karena itu, perlu adanya sebuah analisis tentang penggerombolan emiten berdasarkan fluktuasi harga saham syariah. Berdasarkan karakteristik data harga saham syariah yang merupakan data deret waktu, algoritma penggerombolan yang akan diterapkan adalah agglomerative hierarchical clustering dengan metode jarak dynamic time warping. Parameter jarak terpilih adalah average linkage karena memiliki koefisien korelasi cophenetic tertinggi yaitu 0,643. Jumlah gerombol yang optimal berdasarkan kriteria koefisien silhouette dan rasio keragaman adalah lima gerombol. Setelah dilakukan uji chow breakpoint pada setiap emiten di setiap gerombol, terdapat tiga emiten di gerombol satu, tiga emiten di gerombol dua, satu emiten di gerombol tiga, dan satu emiten di gerombol lima yang tidak signifikan terjadi perubahan struktural pada periode sebelum dan sesudah munculnya pandemi Covid-19., The government policies to break the chain of Covid-19 transmission have an impact on the economic sector in Indonesia. Many workers lost their job and have to find other ways to survive. One of them is by investing in stocks. Indonesia, as a country with a predominantly Islamic population, the sharia capital market, especially sharia stocks is high in demand. However, sharia stocks that have a high return rate also have high risks. One of the factors that make stock risk is high is due to the stock price that is volatile and stochastic. Therefore, it is necessary to cluster the stock issuers based on price pattern. Based on the characteristics of sharia stock price which is time series data, the clustering algorithm that were applied is agglomerative hierarchical clustering with dynamic time warping distance. The distance parameter that were chosen is the average linkage because it has the highest cophenetic correlation coefficient, 0.643. The optimal number of clusters based on the criteria of the silhouette coefficient and diversity ratio is five clusters. According to the chow breakpoint test there were three issuers in cluster one, three issuers in cluster two, one issuer in cluster three, and one issuer in cluster five with no significant structural changes the period before and after the emergence of Covid-19. The test was performed independently on each issuer in all cluster.
Keyword: chow breakpoint test, dynamic time warping, sharia stock, time series clustering

Judul: Hamiltonian formulation for linear waves motion in two layers fluid
Abstrak: The surface wave could be considered as a wave that separates two fluids, namely water and air. Based on this assumption, it is introduced the interfacial wave, a wave between two layers of fluid with different density. The formulation of interfacial waves motion begins with deriving the base equation of irrotational ideal fluid. Furthermore, according to irrotational fluid assumption, the base equation can be stated in velocity potential. In this derivation, the fluids domain is assumed to be restricted by rigid lid boundary conditions, both at the upper and lower limit. Therefore, the interfacial waves motion can be explained in a hamiltonian formulation. In the hamiltonian formulation, total energy is defined as the sum of kinetic and potential energy. The hamiltonian system is obtained from reduction of kinetic energy by using the Dirichlet Neumann Operator. The resulted kinetic energy equation is nonlinear. Therefore, this form is linearized by first part of the Taylor expansion. This linearization gives a dispersion relation of linear wave. Based on this dispersion relation, the phase speed of the linear wave depends on the density ratio of the two layers fluid.
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