What is the feature or improvement you would like to see?

Expose igraph_degree_correlation_vector().

This function computes the $k_{nn}(k)$ function, which can already be computed by knn(), even for a subset of vertices.

In comparison to knn(), igraph_degree_correlation_vector() gives more fine grained control over how in/out degrees are used, but it does not allow restricting the computation to a subset of vertices. This is fine, as such a restriction would rarely be needed and would require a special interpretation.

I recommend exposing this as a new function called knnk().

Use cases for the feature

The $k_{nn}(k)$ function is one of the primary way to characterize degree correlations. This is useful both for teaching and practical network analysis.

References

• https://networksciencebook.com/chapter/7#measuring-degree