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### Spectral Clustering: The Ng-Jordan-Weiss (NJW) Algorithm

- Given a set of objects o1, …, on, and the distance between each pair of objects, dist(oi, oj), find the desired number k of clusters
- Calculate an affinity matrix W, where σ is a scaling parameter that controls how fast the affinity
*Wij*decreases as dist(oi, oj) increases. In NJW, set Wij = 0

\[D_{ii}=\sum_{j=1}^{n}W_{ij}\]

- Derive a matrix A =
*f*(W). NJW defines a matrix D to be a diagonal matrix s.t. Dii is the sum of the i-th row of W, i.e.,

\[W_{ij}=e^{-\frac{dist(o_{i},o_{j})}{\sigma^{2}}}\]

- Then, A is set to

\[A=D^{-\frac{1}{2}}WD^{-\frac{1}{2}}\]

- A spectral clustering method finds the k leading eigenvectors of A
- A vector v is an eigenvector of matrix A if Av = λv, where λ is the corresponding eigen-value
- Using the k leading eigenvectors, project the original data into the new space defined by the k leading eigenvectors, and run a clustering algorithm, such as
*k*-means, to find k clusters - Assign the original data points to clusters according to how the transformed points are assigned in the clusters obtained

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