Abstract
We study random walks in a Hilbert space H and their applications to representations of solutions to Cauchy problems for differential equations whose initial conditions are number-valued functions on the Hilbert space H. Examples of such representations of solutions to various evolution equations in the case of a finite-dimensional space H are given. Measures on a Hilbert space that are invariant with respect to shifts are considered for constructing such representations in infinite-dimensional Hilbert spaces. According to a theorem of A. Weil, there is no Lebesgue measure on an infinite-dimensional Hilbert space. We study a finitely additive analog of the Lebesgue measure, namely, a nonnegative, finitely additive measure λ defined on the minimal ring of subsets of an infinite-dimensional Hilbert space H containing all infinite-dimensional rectangles whose products of sides converge absolutely; this measure is invariant with respect to shifts and rotations in the Hilbert space H. We also consider finitely additive analogs of the Lebesgue measure on the spaces lp, 1 ≤ p ≤ ∞, and introduce the Hilbert space \( \mathcal{H} \) of complex-valued functions on the Hilbert space H that are square integrable with respect to a shift-invariant measure λ. We also obtain representations of solutions to the Cauchy problem for the diffusion equation in the space H and the Schrödinger equation with the coordinate space H by means of iterations of the mathematical expectations of random shift operators in the Hilbert space \( \mathcal{H} \).
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Translated from Itogi Nauki i Tekhniki, Seriya Sovremennaya Matematika i Ee Prilozheniya. Tematicheskie Obzory, Vol. 140, Differential Equations. Mathematical Physics, 2017.
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Sakbaev, V.Z. Random Walks and Measures on Hilbert Space that are Invariant with Respect to Shifts and Rotations. J Math Sci 241, 469–500 (2019). https://doi.org/10.1007/s10958-019-04438-z
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DOI: https://doi.org/10.1007/s10958-019-04438-z
Keywords and phrases
- finitely additive measure
- invariant measure on a group
- random walk
- diffusion equation
- Cauchy problem
- Chernov theorem