arXiv subjects

Kolbjørn Tunstrøm

Publications and source records attributed to Kolbjørn Tunstrøm.

6 recordsLinked to original sources

arXiv2018

Defining the prime numbers prior to the integers: A first-principles approach to the distribution of primes

While the prime numbers have been subject to mathematical inquiry since the ancient Greeks, the accumulated effort of understanding these numbers has - as Marcus du Sautoy recently phrased it - 'not revealed the origins of what makes the primes tick.' Here, we suggest that a resolution to this long-standing conundrum is attainable by defining the primes prior to the natural numbers - as opposed to the standard number theoretical definition of primes where these numbers derive from the natural numbers. The result is a first-principles perspective on the primes that exposes and explains the 'origins' of their distribution and their mathematical properties and provides an intuitive as well as pedagogical approach to the primes with the potential to impact our thinking about these age-old mathematical objects. A few immediate outcomes of this perspective are another proof of the fundamental theorem of arithmetic, a probabilistic model of primes sharing as well as explaining their subrandom correlation structure, and an equivalent formulation of the Riemann hypothesis.

math.GM↗

arXiv2014

Primes in the intervals between primes squared

The set of short intervals between consecutive primes squared has the pleasant---but seemingly unexploited---property that each interval $s_k:=\{p_k^2, \dots,p_{k+1}^2-1\}$ is fully sieved by the $k$ first primes. Here we take advantage of this essential characteristic and present evidence for the conjecture that $π_k \sim |s_k|/ \log p_{k+1}^2$, where $π_k$ is the number of primes in $s_k$; or even stricter, that $y=x^{1/2}$ is both necessary and sufficient for the prime number theorem to be valid in intervals of length $y$. In addition, we propose and substantiate that the prime counting function $π(x)$ is best understood as a sum of correlated random variables $π_k$. Under this assumption, we derive the theoretical variance of $π(p_{k+1}^2)=\sum_{j=1}^k π_j$, from which we are led to conjecture that $|π({x})-\textrm{li}(x)| =O(\sqrt{\textrm{li}(x)})$. Emerging from our investigations is the view that the intervals between consecutive primes squared hold the key to a furthered understanding of the distribution of primes; as evidenced, this perspective also builds strong support in favour of the Riemann hypothesis.

math.NT↗

arXiv2013

The origin of the logarithmic integral in the prime number theorem

We establish why li(x) outperforms x/log x as an estimate for the prime counting function pi(x). The result follows from subdividing the natural numbers into the intervals s_k :={p_k^2,..., p_{k+1}^2-1}, k>=1, each being fully sieved by the k first primes {p_1,..., p_k}. Denoting the number of primes in s_k by pi_k, we show that pi_k |s_k|/log p_{k+1}^2 and that pi(x) li(x) originates as a continuum approximation of the sum sum_k pi_k. In contrast, pi(x) x/log x stems from sieving repeatedly in regions already completed---explaining why x/log x underestimates pi(x). The explanatory potential arising from defining s_k appears promising, evidenced in the last section where we outline further research.

math.NT↗

arXiv2012

Collective dynamics of self-propelled particles with variable speed

Understanding the organization of collective motion in biological systems is an ongoing challenge. In this Paper we consider a minimal model of self-propelled particles with variable speed. Inspired by experimental data from schooling fish, we introduce a power-law dependency of the speed of each particle on the degree of polarization order in its neighborhood. We derive analytically a coarse-grained continuous approximation for this model and find that, while the variable speed rule does not change the details of the ordering transition leading to collective motion, it induces an inverse power-law correlation between the speed or the local polarization order and the local density. Using numerical simulations, we verify the range of validity of this continuous description and explore regimes beyond it. We discover, in disordered states close to the transition, a phase-segregated regime where most particles cluster into almost static groups surrounded by isolated high-speed particles. We argue that the mechanism responsible for this regime could be present in a wide range of collective motion dynamics.

physics.bio-ph↗

arXiv2008

Effective thermostat induced by coarse-graining of SPC water

We investigate how the transport properties of a united atoms fluid with a dissipative particle dynamics thermostat depend on the functional form and magnitude of both the conservative and the stochastic interactions. We demonstrate how the thermostat strongly affects the hydrodynamics, especially diffusion, viscosity, and local escape times. As model system we use SPC water, from which projected trajectories are used to determine the effective interactions in the united atoms model. The simulation results support our argument that the thermostat should be viewed as an integral part of the coarse-grained dynamics, rather than a tool for approaching thermal equilibrium. As our main result we show that the united atoms model with the adjusted effective interactions approximately reproduce the diffusion constant and the viscosity of the underlying detailed SPC water model.

physics.chem-ph↗

arXiv2007

Using force covariance to derive effective stochastic interactions in dissipative particle dynamics

There exist methods for determining effective conservative interactions in coarse grained particle based mesoscopic simulations. The resulting models can be used to capture thermal equilibrium behavior, but in the model system we study do not correctly represent transport properties. In this article we suggest the use of force covariance to determine the full functional form of dissipative and stochastic interactions. We show that a combination of the radial distribution function and a force covariance function can be used to determine all interactions in dissipative particle dynamics. Furthermore we use the method to test if the effective interactions in dissipative particle dynamics (DPD) can be adjusted to produce a force covariance consistent with a projection of a microscopic Lennard-Jones simulation. The results indicate that the DPD ansatz may not be consistent with the underlying microscopic dynamics. We discuss how this result relates to theoretical studies reported in the literature.

cond-mat.soft↗