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[2] Bobrowski D.: On oscillatory and nonoscillatory solutions of certain systems of three ordinary differential equations. In: Poznań.towarz.przyaciól nauk, wydz. nauk.techn., prace komisji nauk podst.stos., tom II, zeszyt 2 (another information has been missed in the accessible reprint)(in Polish).

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[4] Tezin A.M.: Conditions for the existence of cycles of three - dimensional autonomous systems. In: Stud.Diff. Eqns,Gos.Ped.Inst., Kuibyshev 1982, 32-40 (in Russian). MR 0727320

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[6] Wang L., Wang M.-Q.: On the construction of globally asymptotically stable Lyapunov’s functions of a type of nonlinear third-order systems. Acta Math.Appl.Sin. 6, 3 (1983), 309-323 (in Chinese). MR 0748533