Sadahisa Sakata "On the attractivity properties for the equation $x'' + a\left( t \right)f_1 \left( x \right)g_1 \left( {x'} \right)x' + b\left( t \right)f_2 \left( x \right)g_2 \left( {x'} \right)x = e\left( {t,x,x'} \right)$," Proceedings of the Japan Academy, Series A, Mathematical Sciences, Proc. Japan Acad. Ser. A Math. Sci. 57(9), 454-457, (1981)