# scg_group

##### SCG Problem Solver

This function solves the Spectral Coarse Graining (SCG) problem; either exactly, or approximately but faster.

- Keywords
- graphs

##### Usage

```
scg_group(V, nt, mtype = c("symmetric", "laplacian", "stochastic"),
algo = c("optimum", "interv_km", "interv", "exact_scg"), p = NULL,
maxiter = 100)
```

##### Arguments

- V
- A numeric matrix of (eigen)vectors to be preserved by the coarse
graining (the vectors are to be stored column-wise in
`V`

). - nt
- A vector of positive integers of length one or equal to
`length(ev)`

. When`algo`

=optimum ,`nt`

contains the number of groups used to partition each eigenvector separately. When`algo`

is equa - mtype
- The type of semi-projectors used in the SCG. For now
symmetric ,laplacian andstochastic are available. - algo
- The algorithm used to solve the SCG problem. Possible values are
optimum ,interv_km ,interv andexact_scg . - p
- A probability vector of length equal to
`nrow(V)`

.`p`

is the stationary probability distribution of a Markov chain when`mtype`

=stochastic . This parameter is ignored in all other cases. - maxiter
- A positive integer giving the maximum number of iterations of
the k-means algorithm when
`algo`

=interv_km . This parameter is ignored in all other cases.

##### Details

The algorithm `V`

. The running time of this algorithm is $O(\max
nt \cdot m^2)$ for the symmetric and laplacian matrix
problems (i.e. when `mtype`

is `V`

. In all three cases, the memory
usage is $O(m^2)$.

The algorithms `nt[i]`

constant-size bins are used to
partition `V[,i]`

. When `algo`

=

Once a minimizing partition (either exact or approximate) has been found for
each eigenvector, the final grouping is worked out as follows: two vertices
are grouped together in the final partition if they are grouped together in
each minimizing partition. In general the size of the final partition is not
known in advance when `ncol(V)`

>1.

Finally, the algorithm

##### Value

- A vector of
`nrow(V)`

integers giving the group label of each object (vertex) in the partition.

##### 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.

##### See Also

scg-method for a detailed introduction. `scg`

,
`scg_eps`

##### Examples

```
## We are not running these examples any more, because they
## take a long time to run and this is against the CRAN repository
## policy. Copy and paste them by hand to your R prompt if
## you want to run them.
# eigenvectors of a random symmetric matrix
M <- matrix(rexp(10^6), 10^3, 10^3)
M <- (M + t(M))/2
V <- eigen(M, symmetric=TRUE)$vectors[,c(1,2)]
# displays size of the groups in the final partition
gr <- scg_group(V, nt=c(2,3))
col <- rainbow(max(gr))
plot(table(gr), col=col, main="Group size", xlab="group", ylab="size")
## comparison with the grouping obtained by kmeans
## for a partition of same size
gr.km <- kmeans(V,centers=max(gr), iter.max=100, nstart=100)$cluster
op <- par(mfrow=c(1,2))
plot(V[,1], V[,2], col=col[gr],
main = "SCG grouping",
xlab = "1st eigenvector",
ylab = "2nd eigenvector")
plot(V[,1], V[,2], col=col[gr.km],
main = "K-means grouping",
xlab = "1st eigenvector",
ylab = "2nd eigenvector")
par(op)
## kmeans disregards the first eigenvector as it
## spreads a much smaller range of values than the second one
### comparing optimal and k-means solutions
### in the one-dimensional case.
x <- rexp(2000, 2)
gr.true <- scg_group(cbind(x), 100)
gr.km <- kmeans(x, 100, 100, 300)$cluster
scg_eps(cbind(x), gr.true)
scg_eps(cbind(x), gr.km)
```

*Documentation reproduced from package igraph, version 1.0.0, License: GPL (>= 2)*