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Nikita Shulga

Publications and source records attributed to Nikita Shulga.

At least 19 recordsLinked to original sources
arXiv2026

Simultaneous Diophantine approximation on the three-dimensional Veronese curve: the complete Hausdorff dimension story

We determine the Hausdorff dimension of the intersection of the set $\mathcal W_3(λ)$ of simultaneously $λ$-well approximable points with the Veronese curve $\mathcal V_3 \subset \mathbb R^3$ for $λ\ge3/5$, thus completing the full range of $λ$ values. Precisely, we show that for $λ\ge 1/3$, $$ \dim\bigl(\mathcal W_3(λ)\cap\mathcal V_3\bigr)= \max\left\{\frac{2-2λ}{1+λ}, \frac{2}{3(1+λ)}\right\}. $$ To the best of the authors' knowledge, this makes $\mathcal V_3$ the first nondegenerate, non-planar curve with a completely determined Hausdorff dimension theory.

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arXiv2026

The uniform Littlewood conjecture fails on a set of positive Hausdorff dimension

The uniform Littlewood conjecture (ULC), introduced by Bandi, Fregoli and Kleinbock, asserts in the two-number case that $$ \lim_{Q\to\infty} Q\min_{1\le n\le Q}\|nξ\|\,\|nζ\|=0 $$ for all real $ξ,ζ$. It is proven to hold for almost every pair $(ξ,ζ)$. Schleischitz, however, has recently disproved the full statement and showed that the set of counterexamples contains a dense $G_δ$ set. We prove that a set of counterexample pairs with the first coordinate being a badly approximable number has Hausdorff dimension at least $3/2$. We further show that the set of badly approximable numbers $ξ$ for which there exists $ζ$ such that $(ξ,ζ)$ is a counterexample to ULC has full Hausdorff dimension. This contrasts with the classical Littlewood conjecture, for which the set of possible counterexamples is known to have Hausdorff dimension $0$.

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arXiv2026

Nine-distance theorem and growth of best-approximation denominators

We prove a nine-distance theorem for Kronecker sequences on flat three-tori. That is, we show that among the first $N$ orbit points, at most nine distinct positive nearest-neighbour distances occur. This proves the conjecture of Haynes and Marklof. An example of Dettmann shows that nine is optimal. More generally, we prove that on a flat $d$-dimensional torus the number of such distances is at most $2^d+1$. The main tool is a new growth theorem for the denominators $q_1<q_2<\cdots$ of best simultaneous approximations in a $d$-dimensional inner-product space, which is of independent interest. We prove that, whenever $q_{n+2^d}$ is defined, either $q_{n+2^d}\ge2q_{n+1}$, or the indices $1,\ldots,2^d$ can be partitioned into disjoint pairs $\{j,k\}$, $j<k$, such that $q_{n+k}=q_n+q_{n+j}$. In particular, $$ q_{n+2^d}\ge \min\{2q_{n+1},q_n+q_{n+2^{d-1}}\}\ge q_n+q_{n+1}. $$

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arXiv2026

Dirichlet improvability in $L_p$-norms

For a norm $F$ on $\mathbb{R}^2$, we consider the set of $F$-Dirichlet improvable numbers $\mathbf{DI}_F$. In the most important case of $F$ being an $L_p$-norm with $p=\infty$, which is a supremum norm, it is well-known that $\mathbf{DI}_F = \mathbf{BA}\cup \mathbb{Q}$, where $\mathbf{BA}$ is a set of badly approximable numbers. It is also known that $\mathbf{BA}$ and each $\mathbf{DI}_F$ are of measure zero and of full Hausdorff dimension. Using classification of critical lattices for unit balls in $L_p$, we provide a complete and effective characterization of $\mathbf{DI}_p:=\mathbf{DI}_{F^{[p]}}$ in terms of the occurrence of patterns in regular continued fraction expansions, where $F^{[p]}$ is an $L_p$-norm with $p\in[1,\infty)$. This yields several corollaries. In particular, we resolve two open questions by Kleinbock and Rao by showing that the set $\mathbf{DI}_{p}\setminus \mathbf{BA}$ is of full Hausdorff dimension, as well as proving some results about the size of the difference $\mathbf{DI}_{p_1}\setminus \mathbf{DI}_{p_2}$. To be precise, we show that the set difference of Dirichlet improvable numbers in Euclidean norm ($p=2$) minus Dirichlet improvable numbers in taxicab norm ($p=1$) and vice versa, that is $\mathbf{DI}_{2}\setminus \mathbf{DI}_{1}$ and $\mathbf{DI}_{1}\setminus \mathbf{DI}_{2}$, are of full Hausdorff dimension. We also find all values of $p$, for which the set $\mathbf{DI}_p^c\cap\mathbf{BA}$ has full Hausdorff dimension. Finally, our characterization result implies that the number $e$ satisfies $e\in \mathbf{DI}_p$ if and only if $p\in(1,2)\cup(p_0,\infty)$ for some special constant $p_0\approx2.57$.

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arXiv2026

The Dirichlet spectrum with respect to $L_1$ norm is $\left[\frac12,1\right]$

We prove that the one-dimensional Dirichlet spectrum with respect to approximation in $L_1$ norm $\mathbb{D}^{[1]}$ satisfies $$ \mathbb{D}^{[1]}=\left[\frac12,1\right]. $$ This is equivalent to the fact that the Minkowski spectrum $\mathbb M$, associated with the Minkowski diagonal continued fraction, satisfies $$ \mathbb M=\left[\frac14,\frac12\right]. $$ Further, we show that level sets $$ Θ_m=\{α\in(0,1)\setminus\mathbb Q:\mathfrak m(α)=m\}, $$ where $\mathfrak{m}(α)$ is the Minkowski constant of $α$, have Hausdorff dimension strictly greater than $1/2$ for any $m\in(1/4,1/2]$, while $\dim_H Θ_{1/4}=\frac{1}{2}$.

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arXiv2025

Approximation by uniformly distributed sequences

We consider approximation properties of real points by uniformly distributed sequences. Under some assumptions on the approximation functions, we prove a Khintchine-type $0$-$1$ dichotomy law. We establish a new connection between uniform distribution and the ubiquity property. Namely, we show that a bound on the discrepancy of the sequence implies the ubiquity property, which helps to obtain divergence results. We further obtain Hausdorff dimension results for weighted sets. The key tools in proving these results are the weighted ubiquitous systems and weighted mass transference principle introduced recently by Kleinbock \& Wang, and Wang \& Wu respectively.

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arXiv2025

Weighted approximation for limsup sets

Theorems of Khintchine, Groshev, Jarník, and Besicovitch in Diophantine approximation are fundamental results on the metric properties of $Ψ$-well approximable sets. These foundational results have since been generalised to the framework of weighted Diophantine approximation for systems of real linear forms (matrices). In this article, we prove analogues of these weighted results in a range of settings including the $p$-adics (Theorems 7 and 8), complex numbers (Theorems 9 and 10), quaternions (Theorems 11 and 12), and formal power series (Theorems 13 and 14). The key tools in proving the main parts of these results are the weighted ubiquitous systems and weighted mass transference principle introduced recently by Kleinbock--Wang [Adv. Math. (2023)] and Wang--Wu [Math. Ann. (2021)].

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arXiv2025

On a Conjecture of Cusick on a sum of Cantor sets

In 1971 Cusick proved that every real number $x\in[0,1]$ can be expressed as a sum of two continued fractions with no partial quotients equal to $1$. In other words, if we define a set $$ S(k):= \{ x\in[0,1] : a_n(x) \geq k \text{ for all } n\in\mathbb{N} \}, $$ then $$ S(2)+S(2) = [0,1]. $$ He also conjectured that this result is unique in the sense that if you exclude partial quotients from $1$ to $k-1$ with $k\geq3$, then the Lebesgue measure $λ$ of the set of numbers which can be expressed as a sum of two continued fractions with no partial quotients from $\{1,\ldots,k-1\}$ is equal to $0$, that is $$λ\Bigl( S(k)+S(k) \Bigl)= 0 \text{ for }k\geq 3.$$ In this paper, we disprove the conjecture of Cusick by showing that $$ S(k)+S(k) \supseteq \left[0,\frac{1}{k-1}\right]. $$ The proof is constructive and does not rely on ideas from previous works on the topic. We also show the existence of countably many 'gaps' in $S(k)+S(k)$, that is intervals, for which the endpoints lie in $S(k)+S(k)$, while none of the elements in the interior do so. Finally, we prove several results on the sums $$ S(m)+S(n) $$ for $m\neq n$.

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arXiv2024

Metrical properties of finite product of partial quotients in arithmetic progressions

We investigate the dynamics of continued fractions and explore the ergodic behaviour of the products of mixed partial quotients in continued fractions of real numbers. For any function $Φ:\mathbb N\to [2,+\infty)$ and any integer $d\geq 1$, we determine the Lebesgue measure and Hausdorff dimension of the set of real numbers for which the product of partial quotients in arithmetic progressions satisfy $a_n(x)a_{2n}(x)\cdots a_{dn}(x)\geq Φ(n)$ for infinitely many positive integers $n$. Our findings shed light on the size of the set of exceptions to Bourgain's (1988) and Host and Kra's (2005) theorems concerning the convergence of multiple ergodic averages for Gauss dynamical systems. By exploring the Hausdorff dimension of these sets, we gain valuable insights into the behaviour of such exceptions. Overall, our research contributes to a deeper understanding of the dynamics of continued fractions and their connection to the convergence properties of ergodic averages in Gauss dynamical systems.

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arXiv2023

Restricted slowly growing digits for infinite iterated function systems

For an infinite iterated function system $\mathbf{f}$ on $[0,1]$ with an attractor $Λ(\mathbf{f})$ and for an infinite subset $D\subseteq \mathbb{N}$, consider the set \[ \mathbb E(\mathbf{f},D)= \{ x \in Λ(\mathbf{f}): a_n(x)\in D \text{ for all }n\in\mathbb N \text{ and }\lim_{n\to\infty} a_n=\infty\}. \] For a function $φ:\mathbb{N}\to [\min D, \infty)$ such that $φ(n)\to\infty$ as $n\to\infty$, we compute the Hausdorff dimension of the set $$ S(\mathbf{f},D,φ) = \left\{ x\in \E(\mathbf{f},D) : a_n(x)\leq φ(n) \text{ for all } n\in\mathbb N \right\}. $$ We prove that the Hausdorff dimension stays the same no matter how slowly the function $φ$ grows. One of the consequences of our result is the recent work of Takahasi (2023), which only dealt with regular continued fraction expansions. We further extend our result to slowly growing products of (not necessarily consecutive) digits.

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arXiv2023

Radical bound for Zaremba's conjecture

Famous Zaremba's conjecture (1971) states that for each positive integer $q\geq2$, there exists positive integer $1\leq a <q$, coprime to $q$, such that if you expand a fraction $a/q$ into a continued fraction $a/q=[a_1,\ldots,a_n]$, all of the coefficients $a_i$'s are bounded by some absolute constant $\mathfrak k$, independent of $q$. Zaremba conjectured that this should hold for $\mathfrak k=5$. In 1986, Niederreiter proved Zaremba's conjecture for numbers of the form $q=2^n,3^n$ with $\mathfrak k=3$ and for $q=5^n$ with $\mathfrak k=4$. In this paper we prove that for each number $q\neq 2^n,3^n$, there exists $a$, coprime to $q$, such that all of the partial quotients in the continued fraction of $a/q$ are bounded by $ \operatorname{rad}(q)-1$, where $\operatorname{rad}(q)$ is the radical of an integer number, i.e. the product of all distinct prime numbers dividing $q$. In particular, this means that Zaremba's conjecture holds for numbers $q$ of the form $q=2^n3^m, n,m\in\mathbb N \cup \{0\}$ with $\mathfrak k= 5$, generalizing Neiderreiter's result. Our result also improves upon the recent result by Moshchevitin, Murphy and Shkredov on numbers of the form $q=p^n$, where $p$ is an arbitrary prime and $n$ sufficiently large.

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arXiv2023

Complex numbers with a prescribed order of approximation and Zaremba's conjecture

Given $b=-A\pm i$ with $A$ being a positive integer, we can represent any complex number as a power series in $b$ with coefficients in $\mathcal A=\{0,1,\ldots, A^2\}$. We prove that, for any real $τ\geq 2$ and any non-empty proper subset $J(b)$ of $\mathcal A$, there are uncountably many complex numbers (including transcendental numbers) that can be expressed as a power series in $b$ with coefficients in $J(b)$ and with the irrationality exponent (in terms of Gaussian integers) equal to $τ$. One of the key ingredients in our construction is the `Folding Lemma' applied to Hurwitz continued fractions. This motivates a Hurwitz continued fraction analogue of the well-known Zaremba's conjecture. We prove several results in support of this conjecture.

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arXiv2023

Metrical properties of exponentially growing partial quotients

A fundamental challenge within the metric theory of continued fractions involves quantifying sets of real numbers, when represented using continued fractions, exhibit partial quotients that grow at specific rates. For any positive function $Φ$, Wang-Wu theorem (2008) comprehensively describes the Hausdorff dimension of the set \begin{equation*} \EE_1(Φ):=\left\{x\in [0, 1): a_n(x)\geq Φ(n) \ {\rm for \ infinitely \ many} \ n\in \N\right\}. \end{equation*} Various generalisations of this set exist, such as substituting one partial quotient with the product of consecutive partial quotients in the aforementioned set which has connections with the improvements to Dirichlet's theorem, and many other sets of similar nature. Establishing the upper bound of the Hausdorff dimension of such sets is significantly easier than proving the lower bound. In this paper, we present a unified approach to get an optimal lower bound for many known setups, including results by Wang-Wu [Adv. Math., 2008], Huang-Wu-Xu [Israel J. Math. 2020], Bakhtawar-Bos-Hussain [Nonlinearity 2020], and several others, and also provide a new theorem derived as an application of our main result. We do this by finding an exact Hausdorff dimension of the set $$S_m(A_0,\ldots,A_{m-1}) \defeq \left\{ x\in[0,1): \, c_i A_i^n \le a_{n+i}(x) < 2c_i A_i^n,0 \le i \le m-1 \ \text{for infinitely many } n\in\N \right\},$$ where each partial quotient grows exponentially and the base is given by a parameter $A_i>1$. For proper choices of $A_i$'s, this set serves as a subset for sets under consideration, providing an optimal lower bound of Hausdorff dimension in all of them. The crux of the proof lies in introducing of multiple probability measures consistently distributed over the Cantor-type subset of $S_m(A_0,\ldots,A_{m-1})$.

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arXiv2023

Metrical properties of the product of partial quotients with geometric mean in continued fractions

The theory of uniform Diophantine approximation concerns the study of Dirichlet improvable numbers and the metrical aspect of this theory leads to the study of the product of consecutive partial quotients in continued fractions. It is known that the dimension of the set of Dirichlet non-improvable numbers depends upon the number of partial quotients in the product string. However, one can see that the Hausdorff dimension is the same for any number of consecutive partial quotients with a constant gap. This paper is aimed at a detailed analysis on how the Hausdorff dimension changes when there is a linear gap in indices and the number of partial quotients in the product grows. More precisely, let $d\in \N_{\ge 1}, t\in\Z_{\geq 0}$ and $f(n)=dn+t$, we present the detailed Hausdorff dimension analysis of the set\begin{equation*} E_{f}(ψ):=\left\{x\in [0, 1): \sqrt[n]{a_{f(n)}(x)a_{2f(n)}(x)\cdots a_{nf(n)}(x)}\geq ψ(n) \ {\rm for \ infinitely \ many} \ n\in \N\right\}. \end{equation*} It is seen that the dimension is larger if $d$ is larger and $t$ has no contribution to the dimension.

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arXiv2023

Difference of irrationality measure functions

For an irrational number $α\in\mathbb{R}$ we consider its irrationality measure function $$ ψ_α(x) = \min_{1\le q\le x,\, q\in\mathbb{Z}} \| qα\|. $$ It is known for all irrational numbers $α$ and $β$ satisfying $α\pmβ\not\in\mathbb{Z}$, there exist arbitrary large values of $t$ with \begin{equation*} | ψ_α(t) - ψ_β(t) | \geqslant \left( \sqrtτ - 1\right) \cdot \min( ψ_α(t), ψ_β(t) ), \end{equation*} where $τ= \frac{\sqrt{5} + 1}{2}$ and this result is optimal for certain numbers equivalent to $τ$. Here we prove that for all irrational numbers $α$ and $β$, satisfying $α\pmβ\not\in\mathbb{Z}$, such that at least one of them is not equivalent to $τ$, there exist arbitrary large values of $t$ with $$ | ψ_α(t) - ψ_β(t) | \geqslant (\sqrt{\sqrt2+1}-1)\cdot \min( ψ_α(t), ψ_β(t) ). $$ Moreover, we show that the constant on the right-hand side is optimal.

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arXiv2022

A Hausdorff dimension analysis of sets with the product of consecutive vs single partial quotients in continued fractions

We present a detailed Hausdorff dimension analysis of the set of real numbers where the product of consecutive partial quotients in their continued fraction expansion grow at a certain rate but the growth of the single partial quotient is at a different rate. We consider the set \begin{equation*} \FF(Φ_1,Φ_2) \defeq \EE(Φ_1) \backslash \EE(Φ_2)=\left\{x\in[0,1): \begin{split} a_n(x)a_{n+1}(x) & \geqΦ_1(n) \text{\,\, for infinitely many } n\in\N a_{n+1}(x) & <Φ_2(n) \text{\,\, for all sufficiently large } n\in\N \end{split} \right\}, \end{equation*} where $Φ_i:\N\to(0,\infty)$ are any functions such that $\lim\limits_{n\to\infty} Φ_i(n)=\infty$. We obtain some surprising results including the situations when $\FF(Φ_1,Φ_2)$ is empty for various non-trivial choices of $Φ_i$'s. Our results contribute to the metrical theory of continued fractions by generalising several known results including the main result of [Nonlinearity, 33(6):2615--2639, 2020]. To obtain some of the results, we consider an alternate generalised set, which may be of independent interest, and calculate its Hausdorff dimension. One of the main ingredients is in the usage of the classical mass distribution principle; specifically a careful distribution of the mass on the Cantor subset by introducing a new idea of two different types of probability measures.

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arXiv2021

Derivative of the iterations of Minkowski question mark function in special points

For the Minkowski question mark function $?(x)$ we consider derivative of the function $f_n(x) = \underbrace{?(?(...?}_\text{n times}(x)))$. Apart from obvious cases (rational numbers for example) it is non-trivial to find explicit examples of numbers $x$ for which $f'_n(x)=0$. In this paper we present a set of irrational numbers, such that for every element $x_0$ of this set and for any $n\in\mathbb{Z}_+$ one has $f'_n(x_0)=0$.

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arXiv2021

Rational approximations to two irrational numbers

For real $ξ$ we consider the irrationality measure function $ψ_ξ(t) = \min_{1\leqslant q \leqslant t, q\in\mathbb{Z}} || qξ||$, where $||\cdot||$ - distance to the nearest integer. We prove that in the case $α\pmβ\notin\mathbb{Z}$ there exist arbitrary large values of $t$ with $$\Bigl | \frac{1}{ψ_α(t)} - \frac{1}{ψ_β(t)} \Bigl | \geqslant \sqrt5\left(1-\sqrt{\frac{\sqrt5-1}{2}}\right)t.$$ The constant on the right-hand side is optimal.

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