The single linkage clustering method (or the nearest neighbor method) is a method of calculating distance between clusters in hierarchical cluster analysis. The linkage function specifying the distance between two clusters is computed as the minimal object-to-object distance , where objects belong to the first cluster, objects belong to the second cluster. In other words, the distance between two clusters is computed as the distance between the two closest objects in the two clusters.
Mathematically the linkage function – the distance 
between clusters and – is described by the following expression :
where
- is the distance between objects and ;
- and are two sets of objects (clusters)