Geodesic
Meromorphic Transformation to the Birkhoff Standard Form in Small Dimensions
Trudy Matematicheskogo Instituta imeni V.A. Steklova, Solitons, geometry, and topology: on the crossroads, Tome 225 (1999), pp. 87-95

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It is proved that every system of 4 or 5 linear differential equations with two irreducible blocks in a neighborhood of an irregular singular point can be transformed to the Birkhoff standard form with the help of a meromorphic transformation which does not increase the Poincaré rank of the singularity. 
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     author = {A. A. Bolibrukh},
     title = {Meromorphic {Transformation} to the {Birkhoff} {Standard} {Form} in {Small} {Dimensions}},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
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     year = {1999},
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     url = {http://geodesic.mathdoc.fr/item/TM_1999_225_a5/}
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A. A. Bolibrukh. Meromorphic Transformation to the Birkhoff Standard Form in Small Dimensions. Trudy Matematicheskogo Instituta imeni V.A. Steklova, Solitons, geometry, and topology: on the crossroads, Tome 225 (1999), pp. 87-95. http://geodesic.mathdoc.fr/item/TM_1999_225_a5/