Birkhoff, G.; Kotin, L. Positive cyclic systems of linear differential equations. (English) Zbl 0202.08501 J. Differ. Equations 7, 407-416 (1970). PDFBibTeX XMLCite \textit{G. Birkhoff} and \textit{L. Kotin}, J. Differ. Equations 7, 407--416 (1970; Zbl 0202.08501) Full Text: DOI
Birkhoff, G.; Kotin, L. Third order positive cyclic systems of linear differential equations. (English) Zbl 0169.11303 J. Differ. Equations 5, 182-196 (1969). PDFBibTeX XMLCite \textit{G. Birkhoff} and \textit{L. Kotin}, J. Differ. Equations 5, 182--196 (1969; Zbl 0169.11303) Full Text: DOI
Birkhoff, G.; Kotin, L. Integro-differential delay equations of positive type. (English) Zbl 0152.11802 J. Differ. Equations 2, 320-327 (1966). PDFBibTeX XMLCite \textit{G. Birkhoff} and \textit{L. Kotin}, J. Differ. Equations 2, 320--327 (1966; Zbl 0152.11802) Full Text: DOI