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1. Intersections of dual 𝑆𝐿₃-webs

   https://doi.org/10.1090/tran/9349  

   Shen, Linhui; Sun, Zhe; Weng, Daping (August 2025, Transactions of the American Mathematical Society)

   We introduce a topological intersection number for an ordered pair of ${\mathrm{SL}}_3$ -webs on a decorated surface. Using this intersection pairing between reduced $({\mathrm{SL}}_3,\mathcal{A})$ -webs and a collection of $({\mathrm{SL}}_3,\mathcal{X})$ -webs associated with the Fock–Goncharov cluster coordinates, we provide a natural combinatorial interpretation of the bijection from the set of reduced $({\mathrm{SL}}_3,\mathcal{A})$ -webs to the tropical set ${\mathcal{A}}_{{\mathrm{PGL}}_3,\stackrel{^<\#comment/>}{S}}^{+}\left({\mathbb{Z}}^t\right)$ , as established by Douglas and Sun in [Forum Math. Sigma 12 (2024), p. e5, 55]. We provide a new proof of the flip equivariance of the above bijection, which is crucial for proving the Fock–Goncharov duality conjecture of higher Teichmüller spaces for ${\mathrm{SL}}_3$ . 

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2. Random walk on group extensions

   https://doi.org/10.1090/tran/9309  

   Golsefidy, S Alireza; Srinivas, Srivatsa (January 2025, Transactions of the American Mathematical Society)

   We study random walks on various group extensions. Under certain bounded generation and bounded scaled conditions, we estimate the spectral gap of a random walk on a quasi-random-by-nilpotent group in terms of the spectral gap of its projection to the quasi-random part. We also estimate the spectral gap of a random-walk on a product of two quasi-random groups in terms of the spectral gap of its projections to the given factors. Based on these results, we estimate the spectral gap of a random walk on the ${\mathbb{F}}_q$ -points of a perfect algebraic group $\mathbb{G}$ in terms of the spectral gap of its projections to the almost simple factors of the semisimple quotient of $\mathbb{G}$ . These results extend a work of Lindenstrauss and Varjú and an earlier work of the authors. Moreover, using a result of Breuillard and Gamburd, we show that there is an infinite set $\mathcal{P}$ of primes of density one such that, if $k$ is a positive integer and $\mathbb{G}=\mathbb{U}⋊<\#comment/>({\mathrm{SL}}_2{)}_{\mathbb{Q}}^m$ is a perfect group and $\mathbb{U}$ is a unipotent group, then the family of all the Cayley graphs of $\mathbb{G}\left(\mathbb{Z}/\underset{i=1}{\overset{k}{\prod <\#comment/>}}{p}_i\mathbb{Z}\right)$ , $p_i\in <\#comment/>\mathcal{P}$ , is a family of expanders. 

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3. Shadow line distributions

   https://doi.org/10.1090/mcom/4110  

   Balakrishnan, Jennifer; Çiperiani, Mirela; Mazur, Barry; Rubin, Karl (July 2025, Mathematics of Computation)

   Let $E$ be an elliptic curve over $\mathbb{Q}$ with Mordell–Weil rank $2$ and $p$ be an odd prime of good ordinary reduction. For every imaginary quadratic field $K$ satisfying the Heegner hypothesis, there is (subject to the Shafarevich–Tate conjecture) a line, i.e., a free ${\mathbb{Z}}_p$ -submodule of rank $1$ , in $E\left(K\right)\otimes <\#comment/>{\mathbb{Z}}_p$ given by universal norms coming from the Mordell–Weil groups of subfields of the anticyclotomic ${\mathbb{Z}}_p$ -extension of $K$ ; we call it theshadow line. When the twist of $E$ by $K$ has analytic rank $1$ , the shadow line is conjectured to lie in $E\left(\mathbb{Q}\right)\otimes <\#comment/>{\mathbb{Z}}_p$ ; we verify this computationally in all our examples. We study the distribution of shadow lines in $E\left(\mathbb{Q}\right)\otimes <\#comment/>{\mathbb{Z}}_p$ as $K$ varies, framing conjectures based on the computations we have made. 

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4. Algebraic cobordism and a Conner–Floyd isomorphism for algebraic K-theory

   https://doi.org/10.1090/jams/1045  

   Annala, Toni; Hoyois, Marc; Iwasa, Ryomei (March 2024, Journal of the American Mathematical Society)

   We formulate and prove a Conner–Floyd isomorphism for the algebraic K-theory of arbitrary qcqs derived schemes. To that end, we study a stable $\mathrm{\infty <\#comment/>}$ -category of non- ${\mathbb{A}}^1$ -invariant motivic spectra, which turns out to be equivalent to the $\mathrm{\infty <\#comment/>}$ -category of fundamental motivic spectra satisfying elementary blowup excision, previously introduced by the first and third authors. We prove that this $\mathrm{\infty <\#comment/>}$ -category satisfies ${\mathbb{P}}^1$ -homotopy invariance and weighted ${\mathbb{A}}^1$ -homotopy invariance, which we use in place of ${\mathbb{A}}^1$ -homotopy invariance to obtain analogues of several key results from ${\mathbb{A}}^1$ -homotopy theory. These allow us in particular to define a universal oriented motivic ${\mathbb{E}}_{\mathrm{\infty <\#comment/>}}$ -ring spectrum $\mathrm{M}\mathrm{G}\mathrm{L}$ . We then prove that the algebraic K-theory of a qcqs derived scheme $X$ can be recovered from its $\mathrm{M}\mathrm{G}\mathrm{L}$ -cohomology via a Conner–Floyd isomorphism\[ ${\mathrm{M}\mathrm{G}\mathrm{L}}^{\ast <\#comment/>\ast <\#comment/>}\left(X\right){\otimes <\#comment/>}_{\mathrm{L}}\mathbb{Z}\left[{\mathrm{\beta <\#comment/>}}^{\pm <\#comment/>1}\right]\simeq <\#comment/>\mathrm{K}{}^{\ast <\#comment/>\ast <\#comment/>}\left(X\right),$ \]where $\mathrm{L}$ is the Lazard ring and $\mathrm{K}{}^{p,q}\left(X\right)=\mathrm{K}{}_{2q-<\#comment/>p}\left(X\right)$ . Finally, we prove a Snaith theorem for the periodized version of $\mathrm{M}\mathrm{G}\mathrm{L}$ . 

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5. Sums of linear transformations

   https://doi.org/10.1090/tran/9433  

   Conlon, David; Lim, Jeck (July 2025, Transactions of the American Mathematical Society)

   We show that if ${\mathcal{L}}_1$ and ${\mathcal{L}}_2$ are linear transformations from ${\mathbb{Z}}^d$ to ${\mathbb{Z}}^d$ satisfying certain mild conditions, then, for any finite subset $A$ of ${\mathbb{Z}}^d$ , $|{\mathcal{L}}_{1}A+{\mathcal{L}}_{2}A|\ge <\#comment/>{(|det({\mathcal{L}}_1){|}^{1/d}+|det({\mathcal{L}}_2\left){|}^{1/d}\right)}^d|A|-<\#comment/>o\left(|A|\right).$ This result corrects and confirms the two-summand case of a conjecture of Bukh and is best possible up to the lower-order term for certain choices of ${\mathcal{L}}_1$ and ${\mathcal{L}}_2$ . As an application, we prove a lower bound for $|A+\mathrm{\lambda <\#comment/>}\cdot <\#comment/>A|$ when $A$ is a finite set of real numbers and $\mathrm{\lambda <\#comment/>}$ is an algebraic number. In particular, when $\mathrm{\lambda <\#comment/>}$ is of the form $(p/q{)}^{1/d}$ for some $p,q,d\in <\#comment/>\mathbb{N}$ , each taken as small as possible for such a representation, we show that $|A+\mathrm{\lambda <\#comment/>}\cdot <\#comment/>A|\ge <\#comment/>(p^{1/d}+q^{1/d}{)}^d|A|-<\#comment/>o(|A|).$ This is again best possible up to the lower-order term and extends a recent result of Krachun and Petrov which treated the case $\mathrm{\lambda <\#comment/>}=\sqrt{2}$ . 

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