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Ibrahim, S.E. (2011) On the Essential Spectra for Products of the General Quasi-Differential Operators and Their Adjoints. International Journal of Pure and Applied Mathematics, 70, 659-689.

has been cited by the following article:

• TITLE: On the Product of General Differential Operators with Their Point Spectra in the Direct Sum Spaces

  AUTHORS: Sobhy El-Sayed Ibrahim

  KEYWORDS: Product of Quasi-Differential Expressions, Essential Spectra, Joint Field of Regularity, Regularly Solvable Operators, Direct Sum Spaces

  JOURNAL NAME: Journal of Applied Mathematics and Physics, Vol.14 No.6, June 30, 2026

  ABSTRACT: In this paper, the product of quasi-differential expressions τ 1 , τ 2 ,?, τ n each of nth order with complex coefficients and its formal adjoints τ 1 + , τ 2 + ,?, τ n + on any finite number of intervals I p =( a p , b p ) , p=1,?,N are considered in the setting of direct sums of L w p 2 ( a p , b p ) -spaces of functions defined on each of the separate intervals, and a number of results concerning the location of the point spectra and regularity fields of product of differential operators generated by such expressions are obtained. Some of these are extensions or generalizations of those in a symmetric case, and of a general case with one interval case, whilst others are new.