Percolation transition in correlated hypergraphs
Correlations are known to play a crucial role in determining the structure of complex networks. Here we study how their presence affects the computation of the percolation threshold in random hypergraphs. In order to mimic the correlation in real network, we build hypergraphs from a generalized hidden variable ensembles and we study the percolation transition by mapping this problem to the fully connected Potts model with heterogeneous couplings.
In the last decade the topological properties of networks have attracted a large interest [1, 2, 3, 4], mainly driven by the emergence of novel dynamical effects in random processes defined on them [5, 6]. The impact of this research is wide and has important consequences in the domains of biology, socio-economical theories and technological infrastructure design. It has been shown that complex networks can display a universal behavior which strongly affects the dynamics of statistical models built on them.
A major example is provided by the percolation transition [7, 8, 9, 10], one of the most famous emergent collective phenomena that can be defined on complex networks. Its dependence on degree distribution, degree-correlations and directionality of the links has been extensively studied in the last years [11, 12, 13]. In particular, the interplay between topological features and the nature of the percolation transition has been fully investigated within different types of random network ensembles [14, 15, 16, 17, 18, 19, 20]. These constitute null models for networks, each of them being formed by graphs sharing with real complex networks a number of structural features, such as degree distribution or correlations between neighboring components.
Recently, attention has been devoted to the structure of hypergraphs [21, 22], describing, for example, many on-line social and professional communities which collaborate in order to give a semantic structure to a set of data. Among these communities, also named folksonomies, we mention Flickr or CiteULike, whose structure is formed by triplets of users, resources and tags linked together. Such kind of networks show important correlations since the interest of the user and the subject of the resources (for example a picture for Flickr and an article for CiteULike) usually have a strong inter-dependence.
These correlations are responsible for the build-up of communities, after projecting these networks into networks made only by user-user, resource-resource and tag-tag interactions 
In a recent paper , the percolation transition in random uncorrelated hypergraphs was characterized providing a first approximation to the real hypergraphs properties. In this work we extend these results to more general ensembles of correlated hypergraphs which can give a better description of real social communities. We will show how to build null models for hypergraphs based on recent parallel construction introduced for networks [19, 20]. Moreover, we will derive the percolation threshold of correlated hypergraphs by mapping the problem to the solution of a fully connected Potts model with heterogeneous couplings  following the method developed in a recent paper on percolation phase transition in simple networks .
2 Correlated random hypergraphs
To mimic the correlation of real hypergraphs we propose to study randomized ensembles of correlated hypergraphs within the same theoretical framework developed in [19, 20] for networks. In these works it has been shown how is possible to correctly define statistical microcanonical and canonical networks ensembles. In particular they show how to obtain the general relation between random networks ensemble - a collection of graphs with fixed number of nodes and links - and the one formed by nodes linked two by two with probability . In the latter case the number of links fluctuates and has a Poissonian distribution with mean . In the thermodynamic limit it is known that these two different ensembles share the same statistical properties when the external parameters, and , satisfy the following relation . We call the microcanonical ensemble because it satisfies an hard constraint on the extensive number of links - like the energy constraint for usual microcanonical ensemble -. While on the other case the ensemble is canonical, in the sense that the Lagrange multiplier let the number of links fluctuates fixing only its average value to . Generalizing this approach to several properties, like i.e. the degree sequence or a certain division into different communities, it is possible to define complex microcanonical and canonical network ensembles satisfying more stringent constraint [19, 20]. Here we sketch how this general framework can be easily generalized to hypergraphs.
Let us consider a hypergraph formed by nodes of different types linked in groups. For example in Flickr we will have and indicating respectively agents, pictures and tags. We assume also, in order to gain in generality, that each node can be associated to a different feature indicating a given classification of the nodes. Again, in the case of Flickr, the agents can be classified in relation to their interests or age, the tags in relation to their general meaning and the pictures in relation of the type of subject is represented.
A given random correlated ensemble of hypergraphs can be defined as the set of all the hypergraphs which satisfy a number of constraints.
In particular, we choose these constraints to be the number of hyperlinks each node has and the number of hyperlinks bridging set of nodes with different features. Following these prescriptions we can construct microcanonical and canonical hypergraphs ensembles.
2.1 Microcanonical hypergraph ensembles
Let us define a hypergraph by the tensor if the nodes are linked together and otherwise. Let’s call the number of nodes of type . The networks in the microcanonical hypergraph ensemble will then satisfy the following conditions,
with being the hyperdegree of node and with being the number of hyperedges between nodes of features .
2.2 Canonical hypergraph ensembles
By using the statistical mechanics approach described in [19, 20] it can be shown that networks in the canonical hypergraph ensembles, satisfying on average the constraints , can be constructed by assigning a hyperlink with probability
If the constants and the tensor satisfy the following conditions
then the hyperdegrees and the number of hyperedges
are Poisson distributed  with average and given by .
In the limit of hypergraphs with a linking probability independent on the features of the nodes we obtain that the probability becomes
Moreover, we recover the configuration model for the uncorrelated hypergraphs taking the limit . In this case the hyperedge probability be
and this last expression describes uncorrelated hypergraphs whose properties have been studied in .
3 Potts Model and percolation transition
It is a well known result [25, 26, 27] of statistical mechanics that the Potts phase transition in a fully connected systems for the number of colors , describes the bond percolation transition [23, 28] in Erdös Rényi random graphs . Recently a fully connected Potts model with heterogeneous couplings [23, 24] was introduced in order to study the percolation transition in random networks with heterogeneous degree distribution and additional structural properties. Here we show that such a method can be extended to correlated canonical hypergraphs.
Instead of a pure Potts model we consider the following generalized Hamiltonian
where the summation runs over all the -sets that compose the fully connected hypergraph and the variables are Potts spins taking values from . We define for later convenience the vector where is the number of sites of a given type . Introducing an external parameter as the inverse of temperature, the partition function of the Hamiltonian model reads
whose cluster expansion gives the following expression
The value is the number of connected components of the hypergraph and
where the last expression is valid in the small limit. After identifying the coupling
then the sum in (8) is a weighted sum over random hypergraphs in which each link has probability reported in equation (2). If the condition holds, the transition of the Hamiltonian model for coincides with the percolation transition in the random correlated hypergraph ensemble. In the next section we will solve the Potts model providing the percolation condition for hypergraphs in the ensemble that could be written in the form
with the matrix to be determined in the following.
3.1 Solving Potts model
In this subsection we solve the Potts model , in the generic case, for arbitrary and given by . We introduce the order parameter indicating the fraction of nodes of type associated to the ‘hidden variable’ and the feature , having Potts spin equal to
where is the probability distribution to have a node of -type with local properties describes by variables and . We have introduced for convenience the Dirac delta function, with its proper normalization and the Kronecker’s delta defined as
Noticing the symmetry of the Hamiltonian in terms of
we can express the partition function as a summation over these new variables
where the free energy function is given by
The phase transition of the Potts model is determined by the point at which the free energy becomes unstable respect to variation of the order parameters around the symmetric solution
In order to determine the stability of the free energy we evaluate the Hessian of components
where we indicate by the triplets . Making explicitly the calculation, the previous equation (17) reads
with the following definition of . Taking from equation , we obtain that the eigenvalue problem associated to the Hessian matrix is
where and are respectively the eigenvalue and the eigenvector of the problem. The equation can be written as
with the defined as
The symmetric solution becomes unstable when the maximal eigenvalue of the Hessian problem becomes positive. Therefore, in order to determine the critical point of the Potts model for we consider equations and when the eigenvalues vanishes . If we take into account the explicit form of the coupling constant given by Eq. (9), we find that the vector takes the form where the ’s satisfy the linear system of equations
with the matrix given by
Therefore the condition determining the critical point of the Potts model for comes from the vanishing of the determinant, i.e. .
3.2 Simplified cases
In the simplified case in which the linking distribution do not depend on the communities we have that . In fact the coupling constant in the Potts model given by (9) becomes simply
then the solution can be expressed as
Using the definition we get a non null solution for the ’s if and only if the condition is satisfied, with defined as
In the case this condition reduces to the following relation
with the proper identification
The formula (27) is valid in the general case of hypergraphs that show non trivial correlation between nodes. We recover as a special case the percolation condition in the uncorrelated hypergraphs, just remembering the relation between the variables ’s and the mean site connectivity . Therefore, using the previous condition, the ’s are given by
and after some algebra we obtain the condition for the percolation already found in 
Correlations account for the non-trivial structure of complex networks and must play a significant role also in the characterization of hypergraphs describing folksonomies. In this paper we have studied ensembles of correlated hypergraphs which can be used to model the interactions between different types of nodes in real complex hypergraphs. We determined the percolation threshold by mapping this problem to a fully connected Potts model with heterogeneous couplings. Our approach extends the present knowledge on percolation in uncorrelated hypergraph. Future development will link these findings to the study and characterization of real folksonomies and to the analysis of the robustness of the giant component phase against the removal of nodes or hyperedges.
This paper was supported by the project IST STREP GENNETEC contract No.034952 and
by MIUR grant 2007JHLPEZ.
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