We determine the best approximation of an arbitrary power A k of an unbounded self-adjoint operator A in a Hilbert space H on the class {x ∈ D(A r) : ∥A r x∥ ≤ 1}, k < r.
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Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, No. 2, pp. 147–153, February, 2009.
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Babenko, V.F., Bilichenko, R.O. Approximation of unbounded operators by bounded operators in a Hilbert space. Ukr Math J 61, 179–187 (2009). https://doi.org/10.1007/s11253-009-0212-2
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DOI: https://doi.org/10.1007/s11253-009-0212-2