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Jose Maria Grau

Publications and source records attributed to Jose Maria Grau.

12 recordsLinked to original sources
arXiv2018

Computing solutions to the congruence ${1^n + 2^n + \dotsb + n^n\equiv p \pmod{n}}$

It is well-known that the congruence $\sum_{i=1}^{ n} i^{ n} \equiv 1 \pmod{n}$ has exactly five solutions: $\{1,2,6,42,1806\}$. In this work, we characterize the solutions to the congruence $1^n + 2^n + \dotsb + n^n\equiv p \pmod{n}$ for every prime $p$. This characterization leads to an algorithm for computing all such solutions, when there is a finite number of them. More generally, our algorithm enables computing all the solutions below a much higher bound as compared to what can be achieved by a naive exhaustive search.

math.NT↗
arXiv2016

Power sums over commutative and unitary rings

In this paper we compute the sum of the $k$-th powers over any finite commutative unital rings, thus generalizing known results for finite fields, the rings of integers modulo $n$ or the ring of Gaussian integers modulo $n$. As an application we focus on quotient rings of the form $\mathbb{Z}/n\mathbb{Z}[x]/(f(x))$ for any polynomial $f\in\mathbb{Z}[x]$

math.RA↗
arXiv2014

On the diameter of the commuting graph of the full matrix ring over the real numbers

In a recent paper C. Miguel proved that the diameter of the commuting graph of the matrix ring $\mathrm{M}_n(\mathbb{R})$ is equal to $4$ either if $n=3$ or $n>4$. But the case $n=4$ remained open, since the diameter could be $4$ or $5$. In this work we close the problem showing that also in this case the diameter is equal to $4$.

math.RA↗
arXiv2014

Counting invertible sums of squares modulo $n$ and a new generalization of Euler totient function

In this paper we introduce and study a family $Φ_k$ of arithmetic functions generalizing Euler's totient function. These functions are given by the number of solutions to the equation $\gcd(x_1^2+\ldots +x_k^2, n)=1$ with $x_1,\ldots,x_k \in {\mathbb{Z}}/n{\mathbb{Z}}$ which, for $k=2,4$ and $8$ coincide, respectively, with the number of units in the rings of Gaussian integers, quaternions and octonions over ${\mathbb{Z}}/n{\mathbb{Z}}$. We prove that $Φ_k$ is multiplicative for every $k$, we obtain an explicit formula for $Φ_k(n)$ in terms of the prime-power decomposition of $n$ and derive an asymptotic formula for $\sum_{n\le x} Φ_k(n)$. As a tool we investigate the multiplicative arithmetic function that counts the number of solutions to $x_1^2+\ldots +x_k^2\equiv λ$ (mod $n$) for $λ$ coprime to $n$, thus extending an old result that dealt only with the prime $n$ case.

math.NT↗
arXiv2014

On the structure of quaternion rings over $\mathbb{Z}/n \mathbb{Z}$

In this paper we investigate the structure of $\left(\frac{a,b}{\Z{n}}\right)$, the quaternion rings over $\Z{n}$. It is proved that these rings are isomorphic to $\left(\frac{-1,-1}{\Z{n}}\right)$ if $ a \equiv b\equiv -1 \pmod{4}$ or to $\left(\frac{1,1}{\Z{n}}\right)$ otherwise. We also prove that the ring $\left(\frac{a,b}{\Z{n}}\right)$ is isomorphic to $\mathbb{M}_2(\Z{n})$ if and only if $n$ is odd and that all quaternion algebras defined over $\Z{n}$ are isomorphic if and only if $n \not \equiv 0 \pmod{4}$.

math.RA↗
arXiv2014

A von Staudt-type formula for $\displaystyle{\sum_{z\in\mathbb{Z}_n[i]} z^k }$

In this paper we study the sum of powers in the Gaussian integers $\mathbf{G}_k(n):=\sum_{a,b \in [1,n]} (a+b i)^k$. We give an explicit formula for $\mathbf{G}_k(n) \pmod n $ in terms of the prime numbers $p \equiv 3 \pmod 4$ with $p \mid \mid n$ and $p-1 \mid k$, similar to the well known one due to von Staudt for $\sum_{i=1}^n i^k \pmod n$. We apply this formula to study the set of integers $n$ which divide $\mathbf{G}_n(n)$ and compute its asymptotic density with six exact digits: $0.971000\ldots$.

math.NT↗
arXiv2011

Cullen Numbers with the Lehmer Property

Here, we show that there is no positive integer $n$ such that the $n$th Cullen number $C_n=n2^n+1$ has the property that it is composite but $ϕ(C_n)\mid C_n-1$.

math.NT↗
arXiv2011

On a variant of Giuga numbers

In this paper, we characterize the odd positive integers $n$ satisfying the congruence $\sum_{j=1} ^ {n-1} j^{\frac{n-1}{2}}\equiv 0 \pmod n$. We show that the set of such positive integers has an asymptotic density which turns out to be slightly larger than 3/8.

math.NT↗
arXiv2010

An $\tilde{O}(\log^2(N))$ time primality test for Generalized Cullen Numbers

Generalized Cullen Numbers are positive integers of the form $C_b(n):=nb^n+1$. In this work we generalize some known divisibility properties of Cullen Numbers and present two primality tests for this family of integers. The first test is based in the following property of primes from this family: $n^{b^{n}}\equiv (-1)^{b}$ (mod $nb^n+1$). It is stronger and has less computational cost than Fermat's test (for bases $b$ and $n$) and than Miller-Rabin's test (for base $n$). Pseudoprimes for this new test seem to be very scarce, only 4 pseudoprimes have been found among the many millions of Generalized Cullen Numbers tested. We also present a second, more demanding, test for wich no pseudoprimes have been found. This test leads to a "quasi-deterministic" test, running in $\tilde{O}(\log^2(N))$ time, which might be very useful in the search of Generalized Cullen Primes.

math.NT↗
arXiv2010

On the base $b$ expansion of the number of trailing zeroes of $b^k!$

Let us denote by $Z_{b}(n)$ the number of trailing zeroes in the base b expansion of $n!$. In this paper we study the connection between the expression of $\vartheta(b):=\lim_{n\to \infty}Z_{b}(n)/n$ in base $b$, and that of $Z_{b}(b^{k})$. In particular, if $b$ is a prime power, we will show the equality between the $k$ digits of $Z_{b}(b^{k})$ and the first $k$ digits in the fractional part of $\vartheta (b)$. In the general case we will see that this equality still holds except for, at most, the last $\lfloor \log_{b}(k) +3\rfloor $ digits. We finally show that this bound can be improved if $b$ is square-free and present some conjectures about this bound.

math.NT↗