mmsbm: Create an undirected degree-corrected mixed membership stochastic blockmodel object

Description

To specify a degree-corrected mixed membership stochastic blockmodel, you must specify the degree-heterogeneity parameters (via n or theta), the mixing matrix (via k or B), and the relative block propensities (optional, via alpha). We provide defaults for most of these options to enable rapid exploration, or you can invest the effort for more control over the model parameters. We strongly recommend setting the expected_degree or expected_density argument to avoid large memory allocations associated with sampling large, dense graphs.

Usage

mmsbm(
  n = NULL,
  theta = NULL,
  k = NULL,
  B = NULL,
  ...,
  alpha = rep(1, k),
  sort_nodes = TRUE,
  force_pure = TRUE,
  poisson_edges = TRUE,
  allow_self_loops = TRUE
)

Value

An undirected_mmsbm S3 object, a subclass of the undirected_factor_model() with the following additional fields:

theta: A numeric vector of degree-heterogeneity parameters.
Z: The community memberships of each node, a matrix() with k columns, whose row sums all equal one.
alpha: Community membership proportion propensities.
sorted: Logical indicating where nodes are arranged by block (and additionally by degree heterogeneity parameter) within each block.

Arguments

n

(degree heterogeneity) The number of nodes in the blockmodel. Use when you don't want to specify the degree-heterogeneity parameters theta by hand. When n is specified, theta is randomly generated from a LogNormal(2, 1) distribution. This is subject to change, and may not be reproducible. n defaults to NULL. You must specify either n or theta, but not both.

theta

(degree heterogeneity) A numeric vector explicitly specifying the degree heterogeneity parameters. This implicitly determines the number of nodes in the resulting graph, i.e. it will have length(theta) nodes. Must be positive. Setting to a vector of ones recovers a stochastic blockmodel without degree correction. Defaults to NULL. You must specify either n or theta, but not both.

k

(mixing matrix) The number of blocks in the blockmodel. Use when you don't want to specify the mixing-matrix by hand. When k is specified, the elements of B are drawn randomly from a Uniform(0, 1) distribution. This is subject to change, and may not be reproducible. k defaults to NULL. You must specify either k or B, but not both.

B

(mixing matrix) A k by k matrix of block connection probabilities. The probability that a node in block i connects to a node in community j is Poisson(B[i, j]). Must be a square matrix. matrix and Matrix objects are both acceptable. If B is not symmetric, it will be symmetrized via the update B := B + t(B). Defaults to NULL. You must specify either k or B, but not both.

...

Arguments passed on to undirected_factor_model

expected_degree: If specified, the desired expected degree of the graph. Specifying expected_degree simply rescales S to achieve this. Defaults to NULL. Do not specify both expected_degree and expected_density at the same time.

expected_density

If specified, the desired expected density of the graph. Specifying expected_density simply rescales S to achieve this. Defaults to NULL. Do not specify both expected_degree and expected_density at the same time.

alpha

(relative block propensities) Relative block propensities, which are parameters of a Dirichlet distribution. All elments of alpha must thus be positive. Must match the dimensions of B or k. Defaults to rep(1, k), or balanced membership across blocks.

sort_nodes

Logical indicating whether or not to sort the nodes so that they are grouped by block and by theta. Useful for plotting. Defaults to TRUE. When TRUE, nodes are first sorted by block membership, and then by degree-correction parameters within each block. Additionally, pi is sorted in increasing order, and the columns of the B matrix are permuted to match the new order of pi.

force_pure

Logical indicating whether or not to force presence of "pure nodes" (nodes that belong only to a single community) for the sake of identifiability. To include pure nodes, block membership sampling first proceeds as per usual. Then, after it is complete, k nodes are chosen randomly as pure nodes, one for each block. Defaults to TRUE.

poisson_edges

Logical indicating whether or not multiple edges are allowed to form between a pair of nodes. Defaults to TRUE. When FALSE, sampling proceeds as usual, and duplicate edges are removed afterwards. Further, when FALSE, we assume that S specifies a desired between-factor connection probability, and back-transform this S to the appropriate Poisson intensity parameter to approximate Bernoulli factor connection probabilities. See Section 2.3 of Rohe et al. (2017) for some additional details.

allow_self_loops

Logical indicating whether or not nodes should be allowed to form edges with themselves. Defaults to TRUE. When FALSE, sampling proceeds allowing self-loops, and these are then removed after the fact.

Generative Model

There are two levels of randomness in a degree-corrected stochastic blockmodel. First, we randomly choose how much each node belongs to each block in the blockmodel. Each node is one unit of block membership to distribute. This is handled by mmsbm(). Then, given these block memberships, we randomly sample edges between nodes. This second operation is handled by sample_edgelist(), sample_sparse(), sample_igraph() and sample_tidygraph(), depending depending on your desired graph representation.

Block memberships

Let $Z_i$ by a vector on the k dimensional simplex representing the block memberships of node $i$. To generate $z_i$ we sample from a Dirichlet distribution with parameter vector $\alpha$. Block memberships for each node are independent.

Degree heterogeneity

In addition to block membership, the MMSBM also allows nodes to have different propensities for edge formation. We represent this propensity for node $i$ by a positive number $\theta_i$.

Edge formulation

Once we know the block membership vector $z_i, z_j$ and the degree heterogeneity parameters $\theta$, we need one more ingredient, which is the baseline intensity of connections between nodes in block i and block j. This is given by a $k \times k$ matrix $B$. Then each edge $A_{i,j}$ is Poisson distributed with parameter

$$ \lambda_{i, j} = \theta_i \cdot z_i^T B z_j \cdot \theta_j. $$

Examples

Run this code


set.seed(27)

lazy_mmsbm <- mmsbm(n = 1000, k = 5, expected_density = 0.01)
lazy_mmsbm

# sometimes you gotta let the world burn and
# sample a wildly dense graph

dense_lazy_mmsbm <- mmsbm(n = 500, k = 3, expected_density = 0.8)
dense_lazy_mmsbm

# explicitly setting the degree heterogeneity parameter,
# mixing matrix, and relative community sizes rather
# than using randomly generated defaults

k <- 5
n <- 1000
B <- matrix(stats::runif(k * k), nrow = k, ncol = k)

theta <- round(stats::rlnorm(n, 2))

alpha <- c(1, 2, 4, 1, 1)

custom_mmsbm <- mmsbm(
  theta = theta,
  B = B,
  alpha = alpha,
  expected_degree = 50
)

custom_mmsbm

edgelist <- sample_edgelist(custom_mmsbm)
edgelist

# efficient eigendecompostion that leverages low-rank structure in
# E(A) so that you don't have to form E(A) to find eigenvectors,
# as E(A) is typically dense. computation is
# handled via RSpectra

population_eigs <- eigs_sym(custom_mmsbm)
svds(custom_mmsbm)$d

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