arXiv subjects

Naoki Sasakura

Publications and source records attributed to Naoki Sasakura.

At least 19 recordsLinked to original sources

arXiv2026

Joint distributions of eigenvectors of symmetric random tensors

We compute the joint distributions of arbitrary numbers of eigenvectors of real and complex symmetric random tensors by the quantum field theoretical methods which were previously used to compute the mean distributions. We obtain the random matrix representations and the large-dimension asymptotics of the joint distributions. The latter can be expressed by a common function of tensor geometries, extending the universality found for the mean distributions to the joint distributions. Several crosschecks of our results are carried out by Monte Carlo computations.

hep-th↗

arXiv2025

Real eigenvalue/vector distributions of random real antisymmetric tensors

Real eigenpairs of a real antisymmetric tensor of order $p$ and dimension $N$ can be defined as pairs of a real eigenvalue and $p$ orthonormal $N$-dimensional real eigenvectors. We compute the signed and the genuine distributions of such eigenvalues of Gaussian random real antisymmetric tensors by using a quantum field theoretical method. An analytic expression for finite $N$ is obtained for the signed distribution and the analytic large-$N$ asymptotic forms for both. We compute the edge of the distribution for large-$N$, one application of which is to give an upper bound (believed tight) of the injective norm of the random real antisymmetric tensor. We find a large-$N$ universality across various tensor eigenvalue distributions: the large-$N$ asymptotic forms of the distributions of the eigenvalues $z$ of the complex, complex symmetric, real symmetric, and real antisymmetric random tensors are all expressed by $e^{N\,B\, h_p(z_c^2/z^2)+o(N)}$, where the function $h_p(\cdot)$ depends only on the order $p$, while $B$ and $z_c$ differ for each case, $NB$ being the total dimension of the eigenvectors and $z_c$ being determined by the phase transition point of the quantum field theory.

hep-th↗

arXiv2025

Characteristic polynomials of tensors via Grassmann integrals and distributions of roots for random Gaussian tensors

We propose a new definition of characteristic polynomials of tensors based on a partition function of Grassmann variables. This new notion of characteristic polynomial addresses general tensors including totally antisymmetric ones, but not totally symmetric ones. Drawing an analogy with matrix eigenvalues obtained from the roots of their characteristic polynomials, we study the roots of our tensor characteristic polynomial. Unlike standard definitions of eigenvalues of tensors of dimension $N$ giving $\sim e^{{\text{constant}} \, N}$ number of eigenvalues, our polynomial always has $N$ roots. For random Gaussian tensors, the density of roots follows a generalized Wigner semi-circle law based on the Fuss-Catalan distribution, introduced previously by Gurau [arXiv:2004.02660 [math-ph]].

math-ph↗

arXiv2025

Renormalization group-like flows in randomly connected tensor networks

Randomly connected tensor networks (RCTN) are the dynamical systems defined by summing over all the possible networks of tensors. Because of the absence of fixed lattice structure, RCTN is not expected to have renormalization procedures. In this paper, however, we consider RCTN with a real tensor, and it is proven that a Hamiltonian vector flow of a tensor model in the canonical formalism with a positive cosmological constant has the properties which a renormalization group (RG) flow of RCTN would have: The flow has fixed points on phase transition surfaces; every flow line is asymptotically terminated by fixed points at both ends, where an upstream fixed point has higher criticality than a downstream one; the flow goes along phase transition surfaces; there exists a function which monotonically decreases along the flow, analogously to the $a$- and $c$-functions of RG. A complete classification of fixed points is given. Although there are no cyclic flows in the strict sense, these exist, if infinitesimal jumps are allowed near fixed points.

hep-th↗

arXiv2024

Three cases of complex eigenvalue/vector distributions of symmetric order-three random tensors

Random tensor models have applications in a variety of fields, such as quantum gravity, quantum information theory, mathematics of modern technologies, etc., and studying their statistical properties, e.g., tensor eigenvalue/vector distributions, are interesting and useful. Recently some tensor eigenvalue/vector distributions have been computed by expressing them as partition functions of zero-dimensional quantum field theories. In this paper, using the method, we compute three cases of complex eigenvalue/vector distributions of symmetric order-three random tensors, where the three cases can be characterized by the Lie-group invariances, $O(N,\mathbb{R})$, $O(N,\mathbb{C})$, and $U(N,\mathbb{C})$, respectively. Exact closed-form expressions of the distributions are obtained by computing partition functions of four-fermi theories, where the last case is of the "signed" distribution which counts the distribution with a sign factor coming from a Hessian matrix. As an application, we compute the injective norm of the complex symmetric order-three random tensor in the large-$N$ limit by computing the edge of the last signed distribution, obtaining agreement with a former numerical result in the literature.

hep-th↗

arXiv2024

The Edge of Random Tensor Eigenvalues with Deviation

The largest eigenvalue of random tensors is an important feature of systems involving disorder, equivalent to the ground state energy of glassy systems or to the injective norm of quantum states. For symmetric Gaussian random tensors of order 3 and of size $N$, in the presence of a Gaussian noise, continuing the work [arXiv:2310.14589], we compute the genuine and signed eigenvalue distributions, using field theoretic methods at large $N$ combined with earlier rigorous results of [arXiv:1003.1129]. We characterize the behaviour of the edge of the two distributions as the variance of the noise increases. We find two critical values of the variance, the first of which corresponding to the emergence of an outlier from the main part of the spectrum and the second where this outlier merges with the corresponding largest eigenvalue and they both become complex. We support our claims with Monte Carlo simulations. We believe that our results set the ground for a definition of pseudospectrum of random tensors based on $Z$-eigenvalues.

hep-th↗

arXiv2024

Signed eigenvalue/vector distribution of complex order-three random tensor

We compute the signed distribution of the eigenvalues/vectors of the complex order-three random tensor by computing a partition function of a four-fermi theory, where signs are from a Hessian determinant associated to each eigenvector. The issue of the presence of a continuous degeneracy of the eigenvectors is properly treated by a gauge-fixing. The final expression is compactly represented by a generating function, which has an expansion whose powers are the dimensions of the tensor index spaces. A crosscheck is performed by Monte Carlo simulations. By taking the large-$N$ limit we obtain a critical point where the behavior of the signed distribution qualitatively changes, and also the end of the signed distribution. The expected agreement of the end of the signed distribution with that of the genuine distribution provides a few applications, such as the largest eigenvalue, the geometric measure of entanglement, and the best rank-one approximation in the large-$N$ limit.

hep-th↗

arXiv2024

Usefulness of signed eigenvalue/vector distributions of random tensors

Quantum field theories can be applied to compute various statistical properties of random tensors. In particular signed distributions of tensor eigenvalues/vectors are the easiest, which can be computed as partition functions of four-fermi theories. Though signed distributions are different from genuine ones because of extra signs of weights, they are expected to coincide in vicinities of ends of distributions. In this paper, we perform a case study of the signed eigenvalue/vector distribution of the real symmetric order-three random tensor. The correct critical point and the correct end in the large $N$ limit are obtained from the four-fermi theory, for which a method using the Schwinger-Dyson equation is very efficient. Since locations of ends are particularly important in applications, such as the largest eigenvalues and the best rank-one tensor approximations, signed distributions are the easiest and highly useful through the Schwinger-Dyson method.

hep-th↗

arXiv2023

Real eigenvector distributions of random tensors with backgrounds and random deviations

As in random matrix theories, eigenvector/value distributions are important quantities of random tensors in their applications. Recently, real eigenvector/value distributions of Gaussian random tensors have been explicitly computed by expressing them as partition functions of quantum field theories with quartic interactions. This procedure to compute distributions in random tensors is general, powerful and intuitive, because one can take advantage of well-developed techniques and knowledge of quantum field theories. In this paper we extend the procedure to the cases that random tensors have mean backgrounds and eigenvector equations have random deviations. In particular, we study in detail the case that the background is a rank-one tensor, namely, the case of a spiked tensor. We discuss the condition under which the background rank-one tensor has a visible peak in the eigenvector distribution. We obtain a threshold value, which agrees with a previous result in the literature.

hep-th↗

arXiv2023

Exact analytic expressions of real tensor eigenvalue distributions of Gaussian tensor model for small $N$

We obtain exact analytic expressions of real tensor eigenvalue/vector distributions of real symmetric order-three tensors with Gaussian distributions for $N\leq 8$. This is achieved by explicitly computing the partition function of a zero-dimensional boson-fermion system with four-interactions. The distributions are expressed by combinations of polynomial, exponential and error functions as results of feasible complicated bosonic integrals which appear after fermionic integrations. By extrapolating the expressions and also using a previous result, we guess a large-$N$ expression. The expressions are compared with Monte Carlo simulations, and precise and good agreement are obtained with the exact and the large-$N$ expressions, respectively. Understanding the feasibility of the integration is left for future study, which would provide a general-$N$ analytic formula.

hep-th↗

arXiv2022

Real tensor eigenvalue/vector distributions of the Gaussian tensor model via a four-fermi theory

Eigenvalue distributions are important dynamical quantities in matrix models, and it is an interesting challenge to study corresponding quantities in tensor models. We study real tensor eigenvalue/vector distributions for real symmetric order-three random tensors with the Gaussian distribution as the simplest case. We first rewrite this problem as the computation of a partition function of a four-fermi theory with $R$ replicated fermions. The partition function is exactly computed for some small-$N,R$ cases, and is shown to precisely agree with Monte Carlo simulations. For large-$N$, it seems difficult to compute it exactly, and we apply an approximation using a self-consistency equation for two-point functions and obtain an analytic expression. It turns out that the real tensor eigenvalue distribution obtained by taking $R=1/2$ is simply the Gaussian within this approximation. We compare the approximate expression with Monte Carlo simulations, and find that, if an extra overall factor depending on $N$ is multiplied to the the expression, it agrees well with the Monte Carlo results. It is left for future study to improve the approximation for large-$N$ to correctly derive the overall factor.

hep-th↗

arXiv2022

Signed distributions of real tensor eigenvectors of Gaussian tensor model via a four-fermi theory

Eigenvalue distributions are important dynamical quantities in matrix models, and it is a challenging problem to derive them in tensor models. In this paper, we consider real symmetric order-three tensors with Gaussian distributions as the simplest case, and derive an explicit formula for signed distributions of real tensor eigenvectors: Each real tensor eigenvector contributes to the distribution by $\pm 1$, depending on the sign of the determinant of an associated Hessian matrix. The formula is expressed by the confluent hypergeometric function of the second kind, which is obtained by computing a partition function of a four-fermi theory. The formula can also serve as lower bounds of real eigenvector distributions (with no signs), and their tightness/looseness are discussed by comparing with Monte Carlo simulations. Large-$N$ limits are taken with the characteristic oscillatory behavior of the formula being preserved.

hep-th↗

arXiv2022

Splitting-merging transitions in tensor-vectors systems in exact large-$N$ limits

Matrix models have phase transitions in which distributions of variables change topologically like the Gross-Witten-Wadia transition. In a recent study, similar splitting-merging behavior of distributions of dynamical variables was observed in a tensor-vectors system by numerical simulations. In this paper, we study the system exactly in some large-$N$ limits, in which the distributions are discrete sets of configurations rather than continuous. We find cascades of first-order phase transitions for fixed tensors, and first- and second-order phase transitions for random tensors, being characterized by breaking patterns of replica symmetries. The system is of interest across three different subjects at least: The splitting dynamics plays essential roles in emergence of classical spacetimes in a tensor model of quantum gravity; The splitting dynamics automatically detects the rank of a tensor in the tensor rank decomposition in data analysis; The system provides a variant of the spherical $p$-spin model for spin glasses with a new non-trivial parameter. We discuss some implications of the results from these perspectives. The results are compared with some numerical simulations to check the large-$N$ convergence and the assumptions made in the analysis.

hep-th↗

arXiv2022

Emergence of classical spacetimes in canonical tensor model

We study the wave function of a tensor model in the canonical formalism by Hamiltonian Monte Carlo method for Lie group symmetric or nearby values for the argument of the wave function, and show that there emerge Lie-group symmetric semi-classical spacetimes. More precisely, we consider some $SO(n+1)\ (n=1,2,3)$ symmetric values for the tensor argument of the wave function, and show that there emerge discrete $n$-dimensional spheres. A key fact is that there exist two phases, the classical phase and the quantum phase, depending on the values of the argument of the wave function, and emergence of classical spaces above occurs in the former phase, while fluctuations of configurations are too large for such emergence in the latter phase. The transition between the two phases has similarity with the Gross-Witten-Wadia transition, or that between the one-cut and the two-cut solutions in the matrix model. Based on the results, we give some speculations on how spacetimes evolve in the tensor model.

hep-th↗

arXiv2021

Emergence of Lie group symmetric classical spacetimes in canonical tensor model

We analyze a wave function of a tensor model in the canonical formalism, when the argument of the wave function takes Lie group invariant or nearby values. Numerical computations show that there are two phases, which we call the quantum and the classical phases, respectively. In the classical phase, fluctuations are suppressed, and there emerge configurations which are discretizations of the classical geometric spaces invariant under the Lie group symmetries. This is explicitly demonstrated for the emergence of $S^n\ (n=1,2,3)$ for $SO(n+1)$ symmetries by checking the topological and the geometric (Laplacian) properties of the emerging configurations. The transition between the two phases has the form of splitting/merging of distributions of variables, resembling a matrix model counterpart, namely, the transition between one-cut and two-cut solutions. However this resemblance is obscured by a difference of the mechanism of the distribution in our setup from that in the matrix model. We also discuss this transition as a replica symmetry breaking. We perform various preliminary studies of the properties of the phases and the transition for such values of the argument.

hep-th↗

arXiv2021

Counting tensor rank decompositions

The tensor rank decomposition is a useful tool for the geometric interpretation of the tensors in the canonical tensor model (CTM) of quantum gravity. In order to understand the stability of this interpretation, it is important to be able to estimate how many tensor rank decompositions can approximate a given tensor. More precisely, finding an approximate symmetric tensor rank decomposition of a symmetric tensor $Q$ with an error allowance $Δ$ is to find vectors $ϕ^i$ satisfying $\|Q-\sum_{i=1}^R ϕ^i\otimes ϕ^i\cdots \otimes ϕ^i\|^2 \leq Δ$. The volume of all possible such $ϕ^i$ is an interesting quantity which measures the amount of possible decompositions for a tensor $Q$ within an allowance. While it would be difficult to evaluate this quantity for each $Q$, we find an explicit formula for a similar quantity by integrating over all $Q$ of unit norm. The expression as a function of $Δ$ is given by the product of a hypergeometric function and a power function. We also extend the formula to generic decompositions of non-symmetric tensors. The derivation depends on the existence (convergence) of the partition function of a matrix model which appeared in the context of the CTM.

gr-qc↗

arXiv2021

Phase profile of the wave function of canonical tensor model and emergence of large spacetimes

To understand spacetime dynamics in the canonical tensor model of quantum gravity for the positive cosmological constant case, we analytically and numerically study the phase profile of its exact wave function in a coordinate representation, instead of the momentum representation analyzed so far. A saddle point analysis shows that Lie group symmetric spacetimes are strongly favored due to abundance of continuously existing saddle points, giving an emergent fluid picture. The phase profile suggests that spatial sizes grow in "time", where sizes are measured by the tensor-geometry correspondence previously introduced using tensor rank decomposition. Monte Carlo simulations are also performed for a few small $N$ cases by applying a re-weighting procedure to an oscillatory integral which expresses the wave function. The results agree well with the saddle point analysis, but the phase profile is subject to disturbances in a large spacetime region, suggesting existence of light modes there and motivating future computations of primordial fluctuations from the perspective of canonical tensor model.

hep-th↗

arXiv2020

Symmetry enhancement in a two-logarithm matrix model and the canonical tensor model

I study a one-matrix model of a real symmetric matrix with a potential which is a sum of two logarithmic functions and a harmonic one. This two-logarithm matrix model is the absolute square norm of a toy wave function which is obtained by replacing the tensor argument of the wave function of the canonical tensor model (CTM) with a matrix. I discuss a symmetry enhancement phenomenon in this matrix model and show that symmetries and dimensions of emergent spaces are stable only in a phase which exists exclusively for the positive cosmological constant case in the sense of CTM. This would imply the importance of the positivity of the cosmological constant in the emergence phenomena in CTM.

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