K-means++ is an optimization technique used to initialize the K-means algorithm in unsupervised machine learning.

It is often used as it is faster and more accurate compared to regular K-means.

K-means++ initialization differs from regular K-means initialization in that it is based on a probabilistic approach to centroid definition. The algorithm tries to initialize centroids that are far apart from each other.

Following the first, random centroid initialization, the next point is chosen based on a probability that depends on the distance to the first point: the further apart the point is, the more probable it is to be chosen.

This adds complexity in the K-means algorithm but it reduces the probability of a bad initialization and thus to a bad clustering result.

K-means++ has a different seeding technique for centroid definition compared to K-means.

The steps are the following:

- The smallest distance between point x and the nearest centroid is denoted by $D(x)$
- A point $c_1$ is chosen randomly from the dataset
- The point $c_i = x'$ is chosen belonging to the dataset with probability $\frac{D(x')^2}{\sum_{x\in X}{D(x)^2}}$
- Step 3 is repeated until the points are chosen

K-means ++ is said to be $O(log\hspace{1mm}k)$competitive. The *O *notation is used to describe asymptotic behavior of functions. Its objective is to characterize the behavior of a function for high value arguments topics in a simple but rigorous way, in order to be able to compare the behavior of several functions with one another. The *O *notation is commonly used to describe a strict asymptotic limit, but the narrow asymptotic limits are more formally and properly denoted by the letter Θ.

K-means attempts to minimize a "potential" that is calculated as the sum of the squared distances of points from their cluster centers. For any possible clustering scenario, the potential of a K-means++ solution can be at most $8 (log\hspace{1mm}k+2)$

times the potential of the best possible solution. This notation is shortened to $O(log\hspace{1mm}k)$.

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## Further reading

k-means++: The Advantages of Careful Seeding

David Arthur, Sergei Vassilvitskii

Academic paper

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