# Noteworthy, but not so famous conjectures resolved recent years

Conjectures play important role in development of mathematics. Mathoverflow gives an interaction platform for mathematicians from various fields, while in general it is not always easy to get in touch with what happens in the other fields.

Question What are the conjectures in your field proved or disproved (counterexample found) in recent years, which are noteworthy, but not so famous outside your field?

Answering the question you are welcome to give some comment for outsiders of your field which would help to appreciate the result.

Asking the question I keep in mind by "recent years" something like a dozen years before now, by a "conjecture" something which was known as an open problem for something like at least dozen years before it was proved and I would say the result for which the Fields medal was awarded like a proof of fundamental lemma would not fit "not so famous", but on the other hand these might not be considered as strict criteria, and let us "assume a good will" of the answerer.

• What about disproved conjectures—where people have found counterexamples? Are those not worthy of being noted? Feb 3, 2019 at 2:06
• @PeterShor do you mean not worthy of being noted or noteworthy of being... knotted? Well anyway, you could ask a separate (not seperate) question. Maybe mathoverflow.net/questions/138310/… would be a good candidate for an answer to it. Feb 3, 2019 at 4:30
• @PeterShor I mean to INclude disproved conjectures, thank you for your remark, I will edit question accordingly Feb 3, 2019 at 8:53
• @KConrad you are hearly welcome to convert comment to an answer, time borderline 12 years is not strict Feb 3, 2019 at 8:58
• @KConrad That example was the first that came to mind when I saw this question, and I might write something about it if I get the time, as well as Soergel's conjecture whose proof provided many of the needed tools for the counter examples. The linked question also really could do with an updated answer now that a partial replacement for Lusztig's conjecture has been found, though the exact status for primes below $2h-2$ is still very uncertain. Feb 3, 2019 at 8:59

A remarkable example is the Gaussian correlation conjecture (which only recently became the Gaussian correlation inequality). The formulation is very simple:

For arbitrary centered Gaussian measure, any two convex symmetric sets are positively correlated.

It was formulated over 60 years ago (in the above general form, in 1972) and since then had been attacked by many mathematicians. Despite its apparent simplicity, only several partial results had been obtained before its complete proof in 2014.

What is remarkable is that the proof was quite simple and came from a retired statistician Thomas Royen, whose previous scientific output was not very noticeable. Moreover, the article was turned down by some scientists. It seems that the true reasons were that the author was not well known, and the article itself did not look serious (you can find its first non-LaTeX version here). Finally, it was published by some predatory "Far East" journal. Unsurprisingly, it took about two years for the proof to come to the public attention, and for its author to become famous.

Unfortunately, the story brings out some unpleasant features of the scientific community: hypocrisy and prejudice.

More on the story here.

Karim Adiprasito proved the g-conjecture for spheres in a preprint that was posted in December of last year: https://arxiv.org/abs/1812.10454.

This was probably considered the biggest open problem in the combinatorics of simplicial complexes. See Gil Kalai's blog post: https://gilkalai.wordpress.com/2018/12/25/amazing-karim-adiprasito-proved-the-g-conjecture-for-spheres/.

Hinged dissections exist. (See 3-piece dissection of square to equilateral triangle? for an animation of Dudeney's famous equilateral-triangle-to-square hinged dissection.)

Abbott, Timothy G., Zachary Abel, David Charlton, Erik D. Demaine, Martin L. Demaine, and Scott Duke Kominers. "Hinged dissections exist." Discrete & Computational Geometry 47, no. 1 (2012): 150-186. Springer link.

"Abstract. We prove that any finite collection of polygons of equal area has a common hinged dissection. That is, for any such collection of polygons there exists a chain of polygons hinged at vertices that can be folded in the plane continuously without self-intersection to form any polygon in the collection. This result settles the open problem about the existence of hinged dissections between pairs of polygons that goes back implicitly to 1864 and has been studied extensively in the past ten years." The proof is not simple—as hinted by the above figure—but it is constructive.

Konstantin Tikhomirov recently proved that the probability that a random $$n\times n$$ Bernoulli matrix $$M_n$$ with independent $$\pm 1$$ entries, and $$\mathbb{P}[M_{ij}=1]=p,\quad 1\leq i,j\leq n,$$ is singular is $$\mathbb{P}[M_n~\mathrm{is~singular}]=(1-p+o_n(1))^n$$ for any fixed $$p\in (0,1/2].$$

This problem was considered by Komlos, Kahn-Komlos-Szemeredi, Bourgain, Tao-Vu etc., so I am unsure if it qualifies in terms of being not-so-famous.

Nevertheless it was exciting reading about it in Gil Kalai's blog here .

The homological conjectures in commutative algebra using perfectoid methods. A survey on many recent developments written by André can be found here.

In https://arxiv.org/abs/1812.02448, Tadayuki Watanabe announced a disproof of the Smale conjecture in dimension 4. In particular, he shows that the inclusion $$O(5) \hookrightarrow \mathrm{Diff}(S^4)$$ is not a homotopy equivalence. This was the last remaining dimension in which it was not known whether the inclusion $$O(n+1) \hookrightarrow \mathrm{Diff}(S^n)$$ was a homotopy equivalence (it is for $$n \leq 3$$ and it is not for $$n \geq 5$$).

A tensor $$T \in V^{\otimes n} = V \otimes V \otimes \dotsb \otimes V$$ is symmetric if $$T$$ is invariant under permutations of the factors $$V$$ in the tensor product (here $$V$$ is a finite-dimensional vector space). The tensor rank of $$T$$ is the least number of terms in a decomposition of $$T$$ as a sum of simple (aka decomposable, aka pure, aka rank one...) terms, i.e., ones of the form $$v_1 \otimes \dotsb \otimes v_n$$. The symmetric tensor rank of $$T$$ is the least number of terms in a decomposition of $$T$$ as a sum of terms of the form $$v^{\otimes n} = v \otimes v \otimes \dotsb \otimes v$$, i.e., $$v_1 = \dotsb = v_n = v$$. Evidently the symmetric tensor rank of a symmetric tensor is greater than or equal to its tensor rank.

Comon raised the question of whether every symmetric tensor has symmetric tensor rank equal to its tensor rank, say over the complex numbers. It is true for $$n=2$$, which is the statement that the (ordinary matrix) rank of a symmetric matrix is equal to the rank of the quadratic form represented by that matrix. "Comon's conjecture" is the assertion that the answer to Comon's question is positive; it's not completely clear to me whether Comon made this assertion (as opposed to just asking the question), but anyway that name has been used. Besides $$n=2$$, some other cases were known.

Recently Shitov has given a counterexample over the complex numbers.

Shitov, Yaroslav, A counterexample to Comon’s conjecture, SIAM J. Appl. Algebra Geom. 2, No. 3, 428-443 (2018). ZBL1401.15004.

The counterexample is fairly complicated. Shitov takes $$n=3$$ and $$\dim V = 800$$, and constructs a symmetric tensor whose rank is $$903$$, but whose symmetric rank is greater than $$903$$. (The symmetric rank of Shitov's example is not known.) Shitov's counterexample works over the complex numbers. No counterexample is known over the real numbers, or any other field. So Comon's conjecture is still open for the reals and other fields, but I wouldn't bet on it holding.

• When I first read at the end "no real counterexample is known" I thought "wait, you just said that there is a counterexample". Then I read further and understood what you meant by a real counterexample. Feb 16, 2019 at 7:53
• @KConrad Thanks for the feedback. I guess I should try to avoid saying "real X" for "X defined over the real numbers", at least when it's meant for a wider audience. Feb 18, 2019 at 17:19
• Imagine what students starting to learn higher math might think when they hear the term "simple complex Lie group/algebra". I remember in the first week of college being shown Serre's A Course in Arithmetic, which had such a gentle-sounding title and then I was baffled by the first two sentences: "Let $K$ be a field. The image of $\mathbf Z$ in $K$ is an integral domain, hence isomorphic to $\mathbf Z$ or to $\mathbf Z/p\mathbf Z$, where $p$ is prime...". Now it's all obvious, but not at that time. Feb 18, 2019 at 18:43
• @KConrad, make it "an open closed subset of a reduced irreducible representation of a simple complex Lie group". Feb 18, 2019 at 21:59
• Update: Shitov and others including Seigal have produced counterexamples over the real numbers. Sep 18, 2022 at 20:46

“Conway’s knot is not slice”, by Lisa Piccirillo, Annals of Mathematics 191-2 (2020) 581-591, settled a problem (we can call it a conjecture) which was at least four decades old. Read in informal account of the result here: https://www.quantamagazine.org/graduate-student-solves-decades-old-conway-knot-problem-20200519/

Ladner's theorem states that there exist $$\mathsf{NP}$$-intermediate problems when $$\mathsf{P}\neq\mathsf{NP}$$. However, the problem constructed in Ladner's proof is rather 'unnatural'. The question arises of whether any 'natural' examples of problems can be $$\mathsf{NP}$$-intermediate.

The Dichotomy Conjecture of Feder and Vardi (first stated here) states that, under the assumption that $$\mathsf{P}\neq\mathsf{NP}$$, the computational problems known as constraint satisfaction problems (CSPs for short) are either $$\mathsf{NP}$$-complete or belong to $$\mathsf{P}$$.

The consensus in the community (last I knew) is that Dmitriy Zhuk (https://arxiv.org/abs/1704.01914) and Andrei Bulatov (https://arxiv.org/abs/1703.03021) have independently proven the conjecture to be true. Their proofs cap a decades long approach of applying universal algebra to the question.

• Amateur mathematician here. How does the first paragraph relate to the rest of this post? The Dichotomy Conjecture being proved neither proves nor disproves the question of "natural NP-intermediate examples", does it? It proves there are no CSPs, but surely there are "natural" non-CSP problems? Feb 3, 2019 at 13:58
• @BlueRaja You are correct. The stated result essentially says that if we want to find NP-intermediate problems, then we have to look into problems more complicated than CSPs. Feb 3, 2019 at 15:04

S. T. Yau conjectured in the 80's that every compact Riemannian 3-manifold should contain infinitely many different minimal surfaces (smooth, closed). This was proved last year by Antoine Song.

Song built on a long story of breakthroughs in the area by Fernando Marques and Andre Neves using Min-Max theory. Another earlier big result was the solution of the Willmore Conjecture about embedding minimal tori: The Willmore energy $$\int_{\Sigma}H^2$$ of any smoothly immersed torus in $$\mathbf{R}^3$$ is at least $$2\pi^2$$.

Connes' embedding conjecture (from 1976) about the structure of infinite-dimensional von Neumann algebras was shown to be false in the paper

• Zhengfeng Ji, Anand Natarajan, Thomas Vidick, John Wright, Henry Yuen, $$\mathsf{MIP}^*=\mathsf{RE}$$, arXiv:2001.04383.

This was done by a quantum computer science argument, which showed that an interactive proof system with multiple provers sharing quantum entanglement (MIP$$^*$$) could give proofs for any recursively enumerable (RE) language. The computational power of MIP$$^*$$ was a long-standing question in complexity theory, but I don't believe anybody thought it was equal to RE until the connection with Connes' conjecture was discovered fairly recently.

The same paper settles Tsirelsen's problem. Boris Tsirelsen stated a theorem without proof in a 1993 survey paper. It was only much later that he was asked about it, and discovered that the simple proof he thought he had didn't work. He posed it as an open problem in 2006, and this paper shows that Tsirelsen's statement is false.

The connection between these problems was already known, so the linked paper only gives a direct proof of the quantum computer science result.

• Just to clarify: which of these conjectures is "noteworthy, but not so famous"? Nov 23, 2020 at 21:47
• @YemonChoi: Connes' embedding conjecture was the central open problem in the field of von Neumann algebras, but it was not that well known among mathematicians in general. The class MIP* was defined in 2004, along with the question of whether it was equal to an already-known complexity class. I don't think anybody dreamt that it was equal to RE until the connection with Connes' conjecture became clear, just a few years ago. So it may not count as a conjecture (but there were conjectures like MIP*=NEXPTIME). And I should add another equivalent conjecture, Tsirelson's problem, to the answer. Nov 23, 2020 at 22:03

The strong no loop conjecture for quiver algebras $$A$$ states that a simple module $$S$$ with $$Ext_A^1(S,S) \neq 0$$ has infinite projective dimension. It was proven here https://www.sciencedirect.com/science/article/pii/S0001870811002714 . The more general conjecture for Artin algebras is still open.

(The result can be used to check for finite global dimension of endomorphism algebras, see for example Does this algebra have finite global dimension ? (Human vs computer).)

The Audin conjecture in symplectic topology, posed in 1988 by Audin in her famous paper on Lagrangian immersions, asserts that all Lagrangian tori in the standard symplectic vector space have minimal Maslov number 2. This was recently proven by Cieliebak and Mohnke:

https://arxiv.org/abs/1411.1870

That paper nicely summarises the history of the conjecture:

"This question was answered earlier for n = 2 by Viterbo  and Polterovich , in the monotone case for n ≤ 24 by Oh , and in the monotone case for general n by Buhovsky  and by Fukaya, Oh, Ohta and Ono [28, Theorem 6.4.35], see also Damian . A different approach has been outlined by Fukaya . The scheme to prove Audin’s conjecture using punctured holomorphic curves was suggested by Y. Eliashberg around 2001. The reason it took over 10 years to complete this paper are transversality problems in the non-monotone case."

Edit: It occurred to me that this paper is probably now published; indeed it appeared in Inventiones in 2017. Here is the DOI:

https://doi.org/10.1007/s00222-017-0767-8

The following comes directly from Gabriel Peyré's excellent twitter feed:

The Weierstrass function is continuous if $$a<1$$ but nowhere differentiable if $$ab>1$$. The Hausdorff dimension of its graph was conjectured by Mandelbrot in 1977 and proved by Shen in 2016.

The Hall-Paige conjecture, first posed in 1955 by Marshall Hall and L. J. Paige, is the following:

A finite group $$G$$ has a complete mapping if and only if its Sylow $$2$$-subgroups are not cyclic.

Note that a complete mapping is a bijection $$\phi : G \to G$$ such that the function given by $$\psi(g) = g \phi(g)$$ is also a bijection. The above statement was shown to be necessary by Hall and Paige, but its sufficiency remained open until very recently; in 2009, it was shown to be sufficient to only check the cases when $$G$$ is a finite simple group, and the same year all finite simple groups except for $$J_4$$ were shown to satisfy the conjecture. John Bray then dealt with this final case in unpublished work, and Peter Cameron was able to convince him (see this) to publish these noteworthy calculations many years later; the final proof of the Hall-Paige conjecture, together with some consequences of it regarding synchronicity in groups, was written up in 2018 and can be found as a preprint on the arXiv.

Maryanthe Malliaris and Saharon Shelah proved that the cardinal characteristics $$\mathfrak p$$ and $$\mathfrak t$$ are equal, answering a question that goes back at least to the 1970's and probably (with different formulation but the same content) to the 1940's:

• "Cofinality spectrum theorems in model theory, set theory, and general topology." J. Amer. Math. Soc. 29 (2016), 237–297, doi:10.1090/jams830, arXiv:1208.5424, Shelah archive:paper 998).

The definition of $$\mathfrak t$$ is the shortest possible length $$\lambda$$ of a well-ordered sequence $$(A_\xi:\xi<\lambda)$$ of infinite subsets of $$\mathbb N$$ such that (1) $$A_\eta-A_\xi$$ is finite whenever $$\xi<\eta<\lambda$$ and (2) there is no infinite $$B\subseteq\mathbb N$$ with $$B-A_\xi$$ finite for all $$\xi<\lambda$$. That is, $$\mathfrak t$$ is the smallest length of any inextendible decreasing-mod-finite sequence of inifinte subsets of $$\mathbb N$$.

$$\mathfrak p$$ is defined similarly except that, instead of requiring the sequence to decreasing mod finite, one requires only that every finitely many of the $$A$$'s have an infinite intersection.

It is easy to check that $$\aleph_1\leq\mathfrak p\leq\mathfrak t\leq2^{\aleph_0}$$. It was also known previously that if $$\mathfrak p=\aleph_1$$ then $$\mathfrak p=\mathfrak t$$. (I believe this result is due to Judith Roitman, but I can't find a reference now, not even in my chapter of the "Handbook of Set Theory" where this result is Theorem 6.25. Mea culpa.) I think it was expected that $$\mathfrak p<\mathfrak t$$ would turn out to be consistent with ZFC, until Malliaris and Shelah proved otherwise. Not only the theorem but the method of proof was surprising, as it involved ideas from model theory, even though the result is purely set-theoretic.

In number theory, the Sato-Tate conjecture about elliptic curves over $$\mathbf Q$$ was a problem from the 1960s and Serre's conjecture on modularity of odd 2-dimensional Galois representation was a conjecture from the 1970s-1980s. Both were settled around 2008. (For ST conj., the initial proof needed an additional technical hypothesis -- not part of the original conjecture -- of a non-integral $$j$$-invariant, which was later removed in 2011.) For those not familiar with these problems, their solutions build on ideas coming from the proof of Fermat's Last Theorem.

Kiran Kedlaya finished the proof of Deligne's conjecture (1.2.10) made in La conjecture de Weil, II, which is definitely "noteworthy", and perhaps "not so famous" compared to the original Weil conjectures.

Colloquium talk: Companions in etale cohomology.

Annotated reading list for the working seminar at the IAS on the proof.

Tyler Lawson's recent proof that the Brown-Peterson spectrum $$BP$$ at the prime 2 has no $$E_∞$$-ring structure. This was later generalized at odd primes, using similar methods, by Andrew Senger.

The proof proceeds via a detailed study of secondary power operations for ring spectra, which is valuable in itself.

This result suggests that $$BP$$ should have no natural "geometric model", since such models often endow the corresponding cohomology theory with an $$E_∞$$-ring structure.

The Weibel conjecture about negative K-groups was proven in 2018 by Moritz Kerz, Florian Strunk, Georg Tamme.

The conjecture states that if $$X$$ is a Noetherian scheme of Krull dimension $$d$$, the negative K-groups $$K_i(X)$$ vanish when $$i<-d$$. Moreover $$\mathbb{A}^1$$-invariance also holds in that range, that is $$K_i(X)\to K_i(X\times\mathbb{A}^r)$$ is an isomorphism for $$i\le -d$$.

The paper where they solve the conjecture is particularly remarkable because they use methods from derived algebraic geometry to solve a problem with apparently no relation to it.

Manolescu refuted the Triangulation Conjecture. The paper is

Pin(2)-equivariant Seiberg-Witten Floer homology and the Triangulation Conjecture, J. Amer. Math. Soc. 29 (2016), 147-176, doi:10.1090/jams829, arXiv:1303.2354

And you can read a blog post about it at Low Dimensional Topology.

• Is this really "not so famous" (as per the question)? (Also, link-only answers tend to be frowned upon since the link could die in the future.) Feb 14, 2019 at 18:14
• I think it's "not so famous." As a topologist, but not a knot theorist, I could not state this conjecture and only have a very fuzzy recollection of ever hearing about it before. Non topologists probably never have. Jul 2, 2022 at 6:33

Graph theory / Discrete dynamics: In 2007, A. Trahtman proved the Road Coloring Conjecture, which had been posited 37 years earlier by R. Adler and B. Weiss.

Recently, Dasgupta, Kakde and Ventullo proved Gross's conjecture on the value at zero of the $$p$$-adic $$L$$-function constructed by Cassou-Noguès, and Deligne and Ribet. The article, On the Gross-Stark Conjecture, was published in Annals of Mathematics in 2018, and can be found here. Here is the abstract:

In 1980, Gross conjectured a formula for the expected leading term at $$s = 0$$ of the Deligne-Ribet $$p$$-adic $$L$$-function associated to a totally even character $$\psi$$ of a totally real feld $$F$$. The conjecture states that after scaling by $$L(\psi\omega^{-1},0)$$, this value is equal to a $$p$$-adic regulator of units in the abelian extension of $$F$$ cut out by $$\psi\omega^{-1}$$. In this paper, we prove Gross's conjecture.

some important conjectures in matroid theory, for instance the Rota conjecture on excluded minors (by Geelen, Gerards and Whittle, still unpublished, note claiming proof here) and the log-concavity conjecture (also due to Rota) for the characteristic polynomial (arxiv.org/abs/1511.02888). The method of the latter had several applications to solve more problems in matroid theory.

edit: let me add to that Liu's counterexample to the extension space conjecture

A Margulis spacetime is the quotient of the Minkowski space by a free proper orientation-preserving isometric action of a free group of rank at least two.

"Based on a question of Margulis, Drumm–Goldman conjectured in the early 1990s that all Margulis spacetimes should be tame, meaning homeomorphic to the interior of a compact manifold."

In a series of paper, I believe Choi, Drumm, and Goldman, and independently Danciger, Kassel, and Guéritaud resolved this conjecture affirmatively.

1. Topological tameness of Margulis spacetimes, by Suhyoung Choi, William Goldman
2. Tameness of Margulis space-times with parabolics, by Suhyoung Choi, Todd Drumm, William Goldman
3. Geometry and topology of complete Lorentz spacetimes of constant curvature, by Jeffrey Danciger, François Guéritaud, Fanny Kassel
4. Margulis spacetimes via the arc complex, by Jeffrey Danciger, François Guéritaud, Fanny Kassel

This MO question The Riemann zeros and the heat equation describes the Newman conjecture. Very briefly, a deformation parameter is introduced into an integral representation of the Riemann zeta function, creating a function of two variables which satisfies the backward heat equation. Newman made the conjecture that any infinitesimal deformation with this extra parameter destroys the Riemann hypothesis:

"This new conjecture is a quantitative version of the dictum that the Riemann hypothesis, if true, is only barely so."

In 2018, Tao and Rodgers were able to use the connection to PDE and posted a proof of the Newman conjecture on the arXiv. Last week, Alexander Dobner, another student of Tao posted a new, purely analytic (and shorter) proof on the arXiv, writing "One final note we make about our proof is that it reveals that Newman's conjecture holds for completely analytic rather than arithmetic reasons.... Thus, our proof of Newman's conjecture is quite different from the proof of Rodgers and Tao which depends fundamentally on knowledge about the gaps between zeta zeros and hence on the arithmetic structure of the zeta function

In the paper

• Friedrich Hirzebruch, Some Problems on Differentiable and Complex Manifolds, Annals of Mathematics Second Series, Vol. 60, No. 2 (1954) pp. 213-236, doi:10.2307/1969629

Hirzebruch collected problems and questions on smooth and complex manifolds presented at a conference the year prior.

For an almost complex manifold $$M$$ equipped with a hermitian metric, one can form the Laplacian $$\Delta_{\bar{\partial}}$$. Even though $$\bar{\partial}$$ need not square to zero on an almost complex manifold, this Laplacian is an elliptic operator and so the kernel of the Laplacian is finite dimensional if $$M$$ is compact. Denote the dimension of this kernel restricted to $$(p,q)$$ forms by $$h^{p,q}$$.

Problem 20 in Hirzebruch's list, attributed to Kodaira and Spencer, asks the following about compact almost complex manifolds:

Let $$M^n$$ be an almost-complex manifold. Choose an Hermitian structure and consider the numbers $$h^{p,q}$$ defined as above. Is $$h^{p,q}$$ independent of the choice of the Hermitian structure? If not, give some other definition of the $$h^{p,q}$$ of $$M^n$$ which depends only on the almost-complex structure and which generalizes the $$h^{p,q}$$ of a complex manifold.

(Note that in the case of an integrable complex structure, the numbers $$h^{p,q}$$ are metric independent as they are the dimension of the Dolbeault cohomology group $$H_{\bar{\partial}}^{p,q}$$.)

In 2020, Holt and Zhang posted a preprint (update: the paper is now published in Advances in Mathematics)

• Tom Holt, Weiyi Zhang, Harmonic Forms on the Kodaira–Thurston Manifold, arXiv:2001.10962,

showing that the numbers $$h^{p,q}$$ are in general metric-dependent. The underlying manifold they work with is the four-dimensional Kodaira–Thurston nilmanifold.

Turan's book On a new method of analysis and its applications focuses on bounds on power sums. The quantity $$T(m,n)=\inf_{|z_k|=1} \max_{\nu=1,\ldots,m} \left| \sum_{k=1}^n z_k^\nu\right|,$$ for various choices of $$m,n$$ has been of interest since then. The case $$m\sim n^{B}$$ has recently been settled by Andersson, using a character sum estimate due to Katz, in the paper available on arXiv here. The result essentially states that $$T(m,n)\asymp \sqrt{n},$$ if $$m=\lfloor n^B \rfloor,$$ if $$B>1$$ is fixed. This was also an open problem by Montgomery in his Ten lectures on the interface between analytic number theory and harmonic analysis.

Here are two examples from operator theory/operator algebras. Both were open problems for more than forty years. The first example is remarkable because it is was quite well known, it is so simple to state and was elusive for so much time. The second example is notable because of its importance and because it was open since Arveson's seminal "subalgebras" paper.

1. In 2015 Kreg Knese proved that von Neumann's inequality holds for triples of $$3 \times 3$$ contractions (this breakthrough followed important work of Lukasz Kosinski). The introduction to Knese's paper explains it all well (I blogged about it, in case you want a version with some more superlatives).

2. In 2013 Davidson and Kennedy proved the existence of "sufficiently many boundary representations" in every operator system, which was an open problem raised by Arveson in 1969. Here is their paper on arxiv (here is a blog post I wrote on this, geared towards non-specialists). Davidson and Kennedy's solution came five years after Arveson himself settled the problem for separable operator systems (following important work of Dritschel and McCullough), which was also an exciting development.

A "hot spot" on a sufficiently regular domain is an interior extremum of the first nonconstant Neumann eigenfunction of the Laplace operator. The Hot Spots conjecture states that hot spots do not exist on convex planar domains.

Chris Judge and Sugata Mondal have settled the Hot Spots conjecture in the affirmative for all Euclidean triangles: Euclidean triangles have no hot spots, Annals of Mathematics 191-1 (2020) 167-211. (preprint)

This conjecture was the subject of Polymath 7.