# scgSemiProjectors

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##### Semi-Projectors

A function to compute the $L$ and $R$ semi-projectors for a given partition of the vertices.

Keywords
array, graphs
##### Usage
scgSemiProjectors(groups, mtype = c("symmetric", "laplacian",
"stochastic"), p = NULL, norm = c("row", "col"),
sparse = getIgraphOpt("sparsematrices"))
##### Arguments
groups
A vector of nrow(X) or vcount(X) integers giving the group label of every vertex in the partition.
mtype
The type of semi-projectors. For now symmetric, laplacian and stochastic are available.
p
A probability vector of length length(gr). p is the stationary probability distribution of a Markov chain when mtype = stochastic. This parameter is ignored in all other cases.
norm
Either row or col. If set to row the rows of the Laplacian matrix sum up to zero and the rows of the stochastic sum up to one; otherwise it is the columns.
sparse
Logical scalar, whether to return sparse matrices.
##### Details

The three types of semi-projectors are defined as follows. Let $\gamma(j)$ label the group of vertex $j$ in a partition of all the vertices. The symmetric semi-projectors are defined as $$L_{\alpha j}=R_{\alpha j}= \frac{1}{\sqrt{|\alpha|}}\delta_{\alpha\gamma(j)},$$ the (row) Laplacian semi-projectors as $$L_{\alpha j}=\frac{1}{|\alpha|}\delta_{\alpha\gamma(j)}\,\,\,\, \textrm{and}\,\,\,\, R_{\alpha j}=\delta_{\alpha\gamma(j)},$$ and the (row) stochastic semi-projectors as $$L_{\alpha j}=\frac{p_{1}(j)}{\sum_{k\in\gamma(j)}p_{1}(k)}\,\,\,\, \textrm{and}\,\,\,\, R_{\alpha j}=\delta_{\alpha\gamma(j)\delta_{\alpha\gamma(j)}},$$ where $p_1$ is the (left) eigenvector associated with the one-eigenvalue of the stochastic matrix. $L$ and $R$ are defined in a symmetric way when norm = col. All these semi-projectors verify various properties described in the reference.

##### Value

• LThe semi-projector $L$.
• RThe semi-projector $R$.

##### References

D. Morton de Lachapelle, D. Gfeller, and P. De Los Rios, Shrinking Matrices while Preserving their Eigenpairs with Application to the Spectral Coarse Graining of Graphs. Submitted to SIAM Journal on Matrix Analysis and Applications, 2008. http://people.epfl.ch/david.morton

SCG for a detailed introduction. scg, scgNormEps, scgGrouping

##### Aliases
• scgSemiProjectors
##### Examples
# compute the semi-projectors and projector for the partition
# provided by a community detection method
g <- barabasi.game(20, m=1.5)
eb <- edge.betweenness.community(g)
memb <- membership(eb)
lr <- scgSemiProjectors(memb)
#In the symmetric case L = R
tcrossprod(lr$R) # same as lr$R %*% t(lr$R) P <- crossprod(lr$R)  # same as t(lr$R) %*% lr$R
#P is an orthogonal projector
isSymmetric(P)
sum( (P %*% P-P)^2 )

## use L and R to coarse-grain the graph Laplacian
lr <- scgSemiProjectors(memb, mtype="laplacian")
L <- graph.laplacian(g)
Lt <- lr$L %*% L %*% t(lr$R)
## or better lr$L \%*\% tcrossprod(L,lr$R)
rowSums(Lt)
Documentation reproduced from package igraph, version 0.6.5-2, License: GPL (>= 2)

### Community examples

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