Spectral Clustering Algorithm Demystified
Tags: spectral, clustering, ai, machinelearning
Published: 2023-06-20 18:30:00

Spectral clustering is a method of clustering data points based on their similarity or affinity, rather than their distance or compactness. The algorithm uses the eigenvalues and eigenvectors of a similarity matrix to project the data into a lower dimensional space, where the clusters become more separable. The algorithm can handle clusters of arbitrary shapes and sizes, unlike k-means which assumes spherical clusters.

## Working

**Step 1:** Construct a similarity matrix S for the data points, where S[i][j] represents the similarity or affinity between the i-th and j-th data point. The similarity can be measured by various methods, such as Gaussian kernel, k-nearest neighbors, or epsilon-neighborhood.
**Step 2:** Compute the degree matrix D for the similarity matrix S, where D[i][i] is the sum of the i-th row of S, and D[i][j] is zero for i not equal to j. The degree matrix represents the degree of connectivity of each data point.
**Step 3:** Compute the Laplacian matrix L for the similarity matrix S and the degree matrix D, where L = D - S. The Laplacian matrix captures the difference between the degree of a data point and its similarity to other data points.
**Step 4:** Compute the eigenvalues and eigenvectors of the Laplacian matrix L, and sort them in ascending order. Choose the k smallest eigenvalues and their corresponding eigenvectors, where k is the number of clusters desired. The eigenvectors represent the new features that best capture the structure of the data.
**Step 5:** Form a matrix U by stacking the k eigenvectors as columns. Each row of U represents a data point in the new feature space.
**Step 6:** Apply k-means clustering on the rows of U to obtain k clusters.

## Pseudocode

```
# Input: data points X, number of clusters k, similarity measure S
# Output: cluster assignments C

# Step 1: Construct the similarity matrix S
S = similarity_matrix(X, S)

# Step 2: Compute the degree matrix D
D = degree_matrix(S)

# Step 3: Compute the Laplacian matrix L
L = laplacian_matrix(D, S)

# Step 4: Compute the eigenvalues and eigenvectors of L
E, V = eigen(L)

# Sort the eigenvalues and eigenvectors in ascending order
E, V = sort(E, V)

# Choose the k smallest eigenvalues and eigenvectors
E_k = E[:k]
V_k = V[:k]

# Step 5: Form the matrix U by stacking V_k as columns
U = stack(V_k)

# Step 6: Apply k-means clustering on U
C = kmeans(U, k)

# Return the cluster assignments
return C
```

## Advantages

- It can handle clusters with complex shapes and sizes, unlike k-means which assumes spherical clusters.
- It can capture the global structure of the data by using the spectral properties of the similarity matrix.
- It can be easily implemented by using standard linear algebra methods.

## Disadvantages

- It requires choosing k in advance, which can be difficult or arbitrary.
- It is sensitive to the choice of similarity measure, which can affect the quality and number of clusters.
- It can be computationally expensive, as it requires calculating and decomposing a large similarity matrix.

## References and Further Reading

- [Spectral Clustering in Machine Learning - GeeksforGeeks](https://www.geeksforgeeks.org/ml-spectral-clustering/)
- [A Tutorial on Spectral Clustering - arXiv.org](https://arxiv.org/pdf/0711.0189.pdf)
- [Spectral Clustering - GitHub Pages](https://eric-bunch.github.io/blog/spectral-clustering/)

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This will be a multipart series which will follow up with more clustering algorithms with their working, pseudocode, advantages and disadvantages.

Please stay tuned for more such content.

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