Geodesic
Some remarks on almost-positivity of $\psi\text{do}$'s
Bollettino della Unione matematica italiana, Série 8, 1B (1998) no. 1, pp. 187-215.

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Per una classe di operatori pseudodifferenziali a caratteristiche multiple vengono date condizioni necessarie e sufficienti per la validità di stime dal basso «ottimali» 
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Parenti, Cesare; Parmeggiani, Alberto. Some remarks on almost-positivity of $\psi\text{do}$'s. Bollettino della Unione matematica italiana, Série 8, 1B (1998) no. 1, pp. 187-215. http://geodesic.mathdoc.fr/item/BUMI_1998_8_1B_1_a9/

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