On the discrepancy principle for the dynamical systems method.pdf

On the discrepancy principle for the dynamical systems method.pdf

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On the discrepancy principle for the dynamical systems method

a r X i v : m a t h / 0 3 0 2 0 0 1 v 1 [ m a t h .D S ] 3 1 J a n 2 0 0 3 Discrepancy principle for the dynamical systems method Key words: ill-posed problems, dynamical systems method (DSM), discrepancy principle, evolution equations. Math subject classification: 34R30, 35R25, 35R30, 37C35, 37L05, 37N30, 47A52, 47J06, 65M30, 65N21; PACS 02.30.-f, 02.30.Tb, 02.30.Zz,02.60Lj, 02.60.Nm, 02.70.Pt, 05.45.-a A.G. Ramm Mathematics Department, Kansas State University, Manhattan, KS 66506-2602, USA ramm@math.ksu.edu Abstract Assume that Au = f, (1) is a solvable linear equation in a Hilbert space, ||A|| ∞, and R(A) is not closed, so problem (1) is ill-posed. Here R(A) is the range of the linear operator A. A DSM (dynamical systems method) for solving (1), consists of solving the following Cauchy problem: u? = ?u+ (B + ?(t))?1A?f, u(0) = u0, (2) where B := A?A, u? := du dt , u0 is arbitrary, and ?(t) 0 is a continuously differentiable function, monotonically decaying to zero as t → ∞. A.G.Ramm has proved that, for any u0, problem (2) has a unique solution for all t 0, there exists y := w(∞) := limt→∞ u(t), Ay = f , and y is the unique minimal-norm solution to (1). If fδ is given, such that ||f ? fδ|| ≤ δ, then uδ(t) is defined as the solution to (2) with f replaced by fδ. The stopping time is defined as a number tδ such that limδ→0 ||uδ(tδ) ? y|| = 0, and limδ→0 tδ = ∞. A discrepancy principle is proposed and proved in this paper. This principle yields tδ as the unique solution to the equation: ||A(B + ?(t))?1A?fδ ? fδ|| = δ, (3) where it is assumed that ||fδ|| δ and fδ ⊥ N(A ?). For nonlinear monotone A a discrepancy principle is formulated and justified. 1 1 Introduction and statement of the result. Assume that Au = f, (1.1) is a solvable linear equation in a Hilbert space, ||A|| ∞, and R(A) is not closed, so problem (1.1) is ill-posed. Here R(A) is the range of the linear operator A. Without loss of generality, assume that ||A|| ≤ 1. Let y be the unique mini

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