In Gohberg et al. (Classes of linear operators, Theorem 4.2, Chapter XVII. Birkhäuser, Basel, 2003), some sufficient conditions are given so that if A is an unbounded Fredholm linear operator and if B is another (possibly unbounded) linear operator, then their algebraic sum A + B is a Fredholm operator. The main objective here consists of extending the previous result to the case of three unbounded linear operators. Namely, some sufficient conditions are given so that if A , B , C are three unbounded linear operators with A being a Fredholm operator, then their algebraic sum \(A + B + C\) is also a Fredholm operator.
Banach Space Integral Equation Linear Operator Differential Operator Operator Theory
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