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Regularity structure

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Martin Hairer's theory of regularity structures provides a framework for studying a large class of subcritical parabolic stochastic partial differential equations arising from quantum field theory.[1] The framework covers the Kardar–Parisi–Zhang equation, the equation and the parabolic Anderson model, all of which require renormalization in order to have a well-defined notion of solution.

Hairer won the 2021 Breakthrough Prize in mathematics for introducing regularity structures.[2]

Definition

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A regularity structure is a triple consisting of:

  • a subset (index set) of that is bounded from below and has no accumulation points;
  • the model space: a graded vector space , where each is a Banach space; and
  • the structure group: a group of continuous linear operators such that, for each and each , we have .

A further key notion in the theory of regularity structures is that of a model for a regularity structure, which is a concrete way of associating to any and a "Taylor polynomial" based at and represented by , subject to some consistency requirements. More precisely, a model for on , with consists of two maps

,
.

Thus, assigns to each point a linear map , which is a linear map from into the space of distributions on ; assigns to any two points and a bounded operator , which has the role of converting an expansion based at into one based at . These maps and are required to satisfy the algebraic conditions

,
,

and the analytic conditions that, given any , any compact set , and any , there exists a constant such that the bounds

,
,

hold uniformly for all -times continuously differentiable test functions with unit norm, supported in the unit ball about the origin in , for all points , all , and all with . Here denotes the shifted and scaled version of given by

.

References

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  1. ^ Hairer, Martin (2014). "A theory of regularity structures". Inventiones Mathematicae. 198 (2): 269–504. arXiv:1303.5113. Bibcode:2014InMat.198..269H. doi:10.1007/s00222-014-0505-4. S2CID 119138901.
  2. ^ Sample, Ian (2020-09-10). "UK mathematician wins richest prize in academia". The Guardian. ISSN 0261-3077. Retrieved 2020-09-13.