Principal parts and canonical factorisation of hypoelliptic polynomials in two variables
Rendiconti del Seminario Matematico della Università di Padova, Tome 37 (1967), pp. 112-132 
@article{RSMUP_1967__37__112_0,
     author = {Friberg, J.},
     title = {Principal parts and canonical factorisation of hypoelliptic polynomials in two variables},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {112--132},
     year = {1967},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {37},
     mrnumber = {212339},
     zbl = {0168.07802},
     language = {en},
     url = {https://www.numdam.org/item/RSMUP_1967__37__112_0/}
}
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Friberg, J. Principal parts and canonical factorisation of hypoelliptic polynomials in two variables. Rendiconti del Seminario Matematico della Università di Padova, Tome 37 (1967), pp. 112-132. https://www.numdam.org/item/RSMUP_1967__37__112_0/

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