Geodesic
Addition of lower order terms preserving almost hypoellipticity of polynomials
Eurasian mathematical journal, Tome 4 (2013) no. 3, pp. 32-52

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A linear differential operator $P(D)$ with constant coefficients is called almost hypoelliptic if all derivatives $P^{(\nu)}(\xi)$ of the characteristic polynomial $P(\xi)$ can be estimated above via $P(\xi)$. In this paper we describe the collection of lower order terms addition of which to an almost hypoelliptic operator $P(D)$ (polynomial $P(\xi)$) preserves its almost hypoellipticity and its strength. 
@article{EMJ_2013_4_3_a3,
     author = {H. G. Ghazaryan},
     title = {Addition of lower order terms preserving almost hypoellipticity of polynomials},
     journal = {Eurasian mathematical journal},
     pages = {32--52},
     publisher = {mathdoc},
     volume = {4},
     number = {3},
     year = {2013},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EMJ_2013_4_3_a3/}
}
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H. G. Ghazaryan. Addition of lower order terms preserving almost hypoellipticity of polynomials. Eurasian mathematical journal, Tome 4 (2013) no. 3, pp. 32-52. http://geodesic.mathdoc.fr/item/EMJ_2013_4_3_a3/