Hierarchical matrix

Hierarchical matrices, often referred to as H-matrices, are a data structure and mathematical framework used to efficiently represent and compute with large, sparse matrices, particularly those that arise in applications related to numerical analysis, scientific computing, and simulations. The main idea behind H-matrices is to exploit the hierarchical structure of the matrix by grouping data in a way that captures its sparsity while enabling efficient operations like matrix-vector multiplication and matrix-matrix multiplication.