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1. Locally Complete Intersection Maps and the Proxy Small Property

   https://doi.org/10.1093/imrn/rnab041  

   Briggs, Benjamin ; Iyengar, Srikanth B ; Letz, Janina C ; Pollitz, Josh ( April 2021 , International Mathematics Research Notices)

   null (Ed.)

   Abstract It is proved that a map ${\varphi }\colon R\to S$ of commutative Noetherian rings that is essentially of finite type and flat is locally complete intersection if and only if $S$ is proxy small as a bimodule. This means that the thick subcategory generated by $S$ as a module over the enveloping algebra $S\otimes _RS$ contains a perfect complex supported fully on the diagonal ideal. This is in the spirit of the classical result that ${\varphi }$ is smooth if and only if $S$ is small as a bimodule; that is to say, it is itself equivalent to a perfect complex. The geometric analogue, dealing with maps between schemes, is also established. Applications include simpler proofs of factorization theorems for locally complete intersection maps. 

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2. Distributionally Robust Quickest Change Detection using Wasserstein Uncertainty Sets

   Xie, Liyan ; Liang, Yuchen ; Veeravalli, Venugopal V ( May 2024 , PMLR)

   Dasgupta, Sanjoy ; Mandt, Stephan ; Li, Yingzhen (Ed.)

   The problem of quickest detection of a change in the distribution of streaming data is considered. It is assumed that the pre-change distribution is known, while the only information about the post-change is through a (small) set of labeled data. This post-change data is used in a data-driven minimax robust framework, where an uncertainty set for the post-change distribution is constructed. The robust change detection problem is studied in an asymptotic setting where the mean time to false alarm goes to infinity. It is shown that the least favorable distribution (LFD) is an exponentially tilted version of the pre-change density and can be obtained efficiently. A Cumulative Sum (CuSum) test based on the LFD, which is referred to as the distributionally robust (DR) CuSum test, is then shown to be asymptotically robust. The results are extended to the case with multiple post-change uncertainty sets and validated using synthetic and real data examples. 

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3. Adjustments to the law of the wall above an Amazon forest explained by a spectral link

   https://doi.org/10.1063/5.0135697  

   Mortarini, Luca ; Katul, Gabriel G. ; Cava, Daniela ; Dias-Junior, Cleo Quaresma ; Dias, Nelson Luis ; Manzi, Antonio ; Sorgel, Matthias ; Araújo, Alessandro ; Chamecki, Marcelo ( February 2023 , Physics of Fluids)

   Modification to the law of the wall represented by a dimensionless correction function ϕRSL(z/h) is derived using atmospheric turbulence measurements collected at two sites in the Amazon in near-neutral stratification, where z is the distance from the forest floor and h is the mean canopy height. The sites are the Amazon Tall Tower Observatory for z/h∈[1,2.3] and the Green Ocean Amazon (GoAmazon) site for z/h∈[1,1.4]. A link between the vertical velocity spectrum Eww(k) (k is the longitudinal wavenumber) and ϕRSL is then established using a co-spectral budget (CSB) model interpreted by the moving-equilibrium hypothesis. The key finding is that ϕRSL is determined by the ratio of two turbulent viscosities and is given as νt,BL/νt,RSL, where νt,RSL=(1/A)∫0∞τ(k)Eww(k)dk, νt,BL=kv(z−d)u*, τ(k) is a scale-dependent decorrelation time scale between velocity components, A=CR/(1−CI)=4.5 is predicted from the Rotta constant CR=1.8, and the isotropization of production constant CI=3/5 given by rapid distortion theory, kv is the von Kármán constant, u* is the friction velocity at the canopy top, and d is the zero-plane displacement. Because the transfer of energy across scales is conserved in Eww(k) and is determined by the turbulent kinetic energy dissipation rate (ε), the CSB model also predicts that ϕRSL scales with LBL/Ld, where LBL is the length scale of attached eddies to z=d, and Ld=u*3/ε is a macro-scale dissipation length.

    

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4. EXTENDING RESULTS OF MORGAN AND PARKER ABOUT COMMUTING GRAPHS

   https://doi.org/10.1017/S0004972721000332  

   BEIKE, NICOLAS F. ; CARLETON, RACHEL ; COSTANZO, DAVID G. ; HEATH, COLIN ; LEWIS, MARK L. ; LU, KAIWEN ; PEARCE, JAMIE D. ( January 2021 , Bulletin of the Australian Mathematical Society)

   null (Ed.)

   Abstract: Morgan and Parker proved that if G is a group with Z(G)=1, then the connected components of the commuting graph of G have diameter at most 10. Parker proved that if, in addition, G is solvable, then the commuting graph of G is disconnected if and only if G is a Frobenius group or a 2-Frobenius group, and if the commuting graph of G is connected, then its diameter is at most 8. We prove that the hypothesis Z (G) = 1 in these results can be replaced with G' \cap Z(G)=1. We also prove that if G is solvable and G/Z(G) is either a Frobenius group or a 2-Frobenius group, then the commuting graph of G is disconnected. 

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5. SEALING OF THE UNIVERSALLY BAIRE SETS

   https://doi.org/10.1017/bsl.2021.29  

   Sargsyan, Grigor ; Trang, Nam ( October 2021 , The bulletin of symbolic logic)

   A set of reals is universally Baire if all of its continuous preimages in topological spaces have the Baire property. 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 is a type of generic absoluteness condition introduced by Woodin that asserts in strong terms that the theory of the universally Baire sets cannot be changed by set forcings. The 𝖫𝖺𝗋𝗀𝖾𝗌𝗍 𝖲𝗎𝗌𝗅𝗂𝗇 𝖠𝗑𝗂𝗈𝗆 ( 𝖫𝖲𝖠 ) is a determinacy axiom isolated by Woodin. It asserts that the largest Suslin cardinal is inaccessible for ordinal definable surjections. Let 𝖫𝖲𝖠 - 𝗈𝗏𝖾𝗋 - 𝗎𝖡 be the statement that in all (set) generic extensions there is a model of 𝖫𝖲𝖠 whose Suslin, co-Suslin sets are the universally Baire sets. We outline the proof that over some mild large cardinal theory, 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 is equiconsistent with 𝖫𝖲𝖠 - 𝗈𝗏𝖾𝗋 - 𝗎𝖡 . In fact, we isolate an exact theory (in the hierarchy of strategy mice) that is equiconsistent with both (see Definition 3.1). As a consequence, we obtain that 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 is weaker than the theory “ 𝖹𝖥𝖢 + there is a Woodin cardinal which is a limit of Woodin cardinals.” This significantly improves upon the earlier consistency proof of 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 by Woodin. A variation of 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 , called 𝖳𝗈𝗐𝖾𝗋 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 , is also shown to be equiconsistent with 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 over the same large cardinal theory. We also outline the proof that if V has a proper class of Woodin cardinals, a strong cardinal, and a generically universally Baire iteration strategy, then 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 holds after collapsing the successor of the least strong cardinal to be countable. This result is complementary to the aforementioned equiconsistency result, where it is shown that 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 holds in a generic extension of a certain minimal universe. This theorem is more general in that no minimal assumption is needed. A corollary of this is that 𝖫𝖲𝖠 - 𝗈𝗏𝖾𝗋 - 𝗎𝖡 is not equivalent to 𝖲𝖾𝖺𝗅𝗂𝗇𝗀 . 

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