Determining factors and hypoellipticity of ordinary differential operators with double "characteristics"
Colloque International C.N.R.S. sur les équations aux dérivées partielles linéaires, Astérisque, no. 2-3 (1973), pp. 197-216. 
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Helffer, Bernard; Kannai, Yakar. Determining factors and hypoellipticity of ordinary differential operators with double "characteristics", dans Colloque International C.N.R.S. sur les équations aux dérivées partielles linéaires, Astérisque, no. 2-3 (1973), pp. 197-216. http://archive.numdam.org/item/AST_1973__2-3__197_0/

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[2] A. R. Forsyth "Theory of differential equations" part III, vol. IV, Cambridge, 1902 | JFM

[3] E. L. Ince "Ordinary differential equations" London, 1927 | JFM

[4] Y. Kannai ''Hypoelliptic ordinary differential operators" Israel J. of Math. 13 (to appear) | MR | Zbl