MR0203182 Hsieh, Po-fang and Sibuya, Yasutaka Po-Fang HSIEH and Yasutaka SIBUYA Note on regular perturbations of linear ordinary differential equations at irregular singular points Funkcialaj Ekvacioj. Serio Internacia 8 1966 99--108 http://fe.math.kobe-u.ac.jp/FE/Free/vol08/fe08-7.pdf http://fe.math.kobe-u.ac.jp/FE/FE_pdf_with_bookmark/FE01-10-en_KML/fe08-099-108/fe08-099-108.pdf http://www.ams.org/mathscinet-getitem?mr=MR0203182 fe08-099-108 1966 Note on Regular Perturbations of Linear Ordinary Differential Equations at Irregular Singular Points HSIEH, Po-Fang; SIBUYA, Yasutaka HSIEH, Po-Fang; SIBUYA, Yasutaka
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