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$%A. Favini, R. Labbas, K. Lemrabet, and B.-K. Sadallah, Study of a complete abstract
differential equation of elliptic type
with variable operator coefficients, I, Rev. Mat. Complut. 21 (2008), no. 1, 89–133.%$
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ABSTRACT
     The aim of this first work is the resolution of an abstract complete second order
     differential equation of elliptic type with variable operator coefficients set in a
     small length interval. We obtain existence, uniqueness and maximal regularity
     results under some appropriate differentiability assumptions combining those of
     "On the abstract evolution equation of parabolic type" (Yagi, 1977) and
		"Sommes d'opérateurs linéaires et équations différentielles 
		opérationnelles" (Da Prato-Grisvard, 1975). An example for the
     Laplacian in a regular domain of  will illustrate the theory. A forthcoming
     work (Part II) will complete the present one by the study of the Steklov-Poincaré
     operator related to this equation when the length
 will illustrate the theory. A forthcoming
     work (Part II) will complete the present one by the study of the Steklov-Poincaré
     operator related to this equation when the length  of the interval tends to
     zero.
 of the interval tends to
     zero.
     
Key words: abstract differential equations of second order, variable operator coefficients,
     mixed boundary conditions, maximal regularity, compatibility conditions.
2000 Mathematics Subject Classification: 34G10, 34K10,34K30, 35J25, 44A45, 47D03.