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: Pengaruh tingkat interaksi pemangsaan dan waktu tunda pada model mangsa-pemangsa dengan pemanenan mangsa konstan
Abstrak: Model dalam tulisan ini disusun dari modifikasi model mangsa-pemangsa Lotka-Volterra dengan mempertimbangkan waktu tunda dan pemanenan konstan terhadap populasi mangsa. Dalam model ini diperoleh tiga titik tetap, dengan titik tetap ketiga merupakan titik tetap yang menjaga koeksistensi mangsa dan pemangsa. Untuk model tanpa waktu tunda dan pemanenan, dengan meningkatkan nilai interaksi pemangsaan pada nilai tertentu akan mengubah kestabilan titik tetap ketiga dari simpul stabil menjadi spiral stabil. Jika pemanenan konstan ditambahkan dalam model tersebut, akan diperoleh nilai interaksi pemangsaan yang akan mengubah kondisi spiral stabil menjadi spiral tak stabil pada titik tetap ketiga. Jika populasi pemangsa dalam interaksi pemangsaan ditambahkan waktu tunda tertentu maka kestabilan sistem mangsa-pemangsa ini akan berubah dari spiral stabil menjadi spiral tak stabil, walaupun nilai interaksi pemangsaan tidak dinaikkan. Dalam kondisi terjadinya perubahan dari spiral stabil menjadi spiral tak stabil, dalam sistem akan muncul limit cycle dan terjadi bifurkasi Hopf.
Keyword: Lotka-Volterra model, prey-predator, constant harvesting, predator rate interaction, time delay, 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