Judul: Perilaku Dinamis Model Mangsa-Pemangsa Tipe Gause yang Diperumum dengan Waktu Tunda Pemanenan Konstan
Abstrak: There are several mathematical models to describe prey-predator events. One model that has many applications is the generalized Gause type prey-predator model by considering a time delay and a constant harvesting parameter in both prey-predator populations. We performed stability analysis to both models without time delay and with time delay. For the model without time delay, we obtained three equilibrium points with one is spiral stable, while model with time delay possesses equilibrium points which can be either spiral stable or spiral unstable. In addition to the model with time delay, when the value of time delay increases, this causes the appearance of a limit-cycle and supercritical Hopf bifurcation occurs when the equilibrium stability change from spiral stable to spiral unstable.
Keyword: Bogor Agricultural University, time delay, supercritical Hopf bifurcation, generalized Gause type prey-predator model

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: 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