MathPHP\LinearAlgebra\Matrix::getDiagonalElements PHP Method

    public function getDiagonalElements() : array
    {
        $diagonal = [];
        if ($this->isSquare()) {
            for ($i = 0; $i < $this->m; $i++) {
                $diagonal[] = $this->A[$i][$i];
            }
        }
        return $diagonal;
    }

 /**
  * Regression Leverages
  * A measure of how far away the independent variable values of an observation are from those of the other observations.
  * https://en.wikipedia.org/wiki/Leverage_(statistics)
  *
  * Leverage score for the i-th data unit is defined as:
  * hᵢᵢ = [H]ᵢᵢ
  * which is the i-th diagonal element of the project matrix H,
  * where H = X⟮XᵀX⟯⁻¹Xᵀ where X is the design matrix.
  *
  * @return array
  */
 public function leverages() : array
 {
     return $this->reg_P->getDiagonalElements();
 }