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://www.numdam.org/item/AST_1973__2-3__197_0/

[1] F. T. Cope "Formal solutions of irregular linear differential equations I" Amer. J. Math. 56 (1934), 411-437 | DOI | JFM | MR

[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