Judul: Bifurkasi Hopf pada Model Mangsa Pemangsa dengan Waktu Tunda dan Tingkat Pemanenan Konstan
Abstrak: In this paper we discussed a predator-prey model by considering a time delay to predator population and constant rate of harvesting in both predator and prey populations. We performed stability analysis to both models with and without time delay. For that without time delay we obtained two equilibrium points which are saddle for the case and spiral stable or node stable in another case. For the case of model with time delay, there are two equilibrium points which are saddle and spiral stable or saddle and spiral unstable in another case. Time delay parameter may result in the existence of limit cycle and Hopf bifurcation.
Keyword: Bogor Agricultural University (IPB), Hopf bifurcation, constant rate of harvesting, time delay, predator-prey

Judul: Bifurkasi Hopf pada Model Mangsa-Pemangsa Holling-Tanner Tipe II
Abstrak: In this paper a mathematical prey-predator model of Holling-Tanner type II and the existence of Hopf bifurcation were studied. In this model, three fixed points are obtained, in which one of them is a saddle point. The prey-predator population dynamics was simulated based on four cases, by increasing the interaction value of prey and predator. In the third case, the stable spiral changed into an unstable spiral and also observed the presence of limit cycles. This is known as Hopf bifurcation. Furthermore, for the fourth case both populations were almost extincted due to the increase of the interaction rate. Generally, it can be concluded that increasing the value of prey and predator interaction rate would change the stability of population. The prey-predator population will be stable when the prey and predator interaction rate is low, whereas the higher interaction rate would cause the extinction of both prey and predator populations
Keyword: prey-predator., limit cycle, Holling-Tanner type II, Hopf bifurcation

Judul: Elektrodeposisi Film Tipis Semikonduktor Cu2ZnSnS4 (CZTS) pada Substrat Kaca ITO
Abstrak: Semiconductor Cu2ZnSnS4 (CZTS) thin film that can be used as absorber layers on solar cell module was made via electrodeposition method. CZTS thin film was electrodeposited on the surface of a silicate glass coated with a thin layer of indium tin oxide (ITO). Deposition process was done using potentiostatic mode at room temperature with a potential of 1.05 V for 30, 45, and 60 minutes, followed with annealing the film at 2 different temperatures, 500 ºC using furnace and 180 ºC using hot plate, both treatments for 45 minutes in N2/H2S atmosphere. The X-ray difractograms were analyzed using Match 2 program and showed that the CZTS thin films had kesterit crystalline structure, with a low percentage of CZTS phase. The thin film also contained secondary phases such as Cu2S, ZnS, and SnS. The obtained CZTS thin films had a thickness between 1.039 and 1.676 μm. Absorption study showed that the band gap energy of CZTS thin films which was annealed using furnace ranged from 1.50 to 1.52 eV, whereas that annealed using the hot plate was not observed. The band gap energy value showed that CZTS thin films can be used as sunlight absorber layers on solar cell. The formation of CZTS crystal film on this investigation was effectively performed using furnace annealing process at a temperature of 500 ºC with deposition time of 45 minutes.
Keyword: Thin film, Electrodeposition, Cu2ZnSnS4