Understand and code the R² metric (goodness of fit) in C++. A must-know concept for evaluating regression models.

Let A be a complex diagonalizable matrix with two distinct nonzero eigenvalues λ and μ, the Yang-Baxter-like matrix equation AXA = XAX is reconsidered. We correct and improve the results in Shen et al ...

In response to recent elaborations of the community matrix concept, I argue that nonlinear (higher-order) interactions between species are probably very important in many competitive relationships.