Let A be a self-adjoint
operator, not necessarily bounded, in the Hilbert space H, with resolution of the
identity Eλ. Define h(t,A) = ∫
−∞∞h(t,λ)dEλ. It is shown that as 𝜖 → 0+ the
solution of the abstract problem 𝜖U𝜖′′ + bU𝜖′ + h(t,A)U𝜖 = 0,U𝜖(0) = x0,U𝜖′(0) = x1
tends in the norm of H to the solution of bU0′ + h(t,A)U0 = 0,U0(0) = x0 for data
x0,x1 in a dense subset of H.
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