public function regressionVariance($x) { $X = $this->createDesignMatrix($x); $⟮XᵀX⟯⁻¹ = $this->⟮XᵀX⟯⁻¹; $M = $X->multiply($⟮XᵀX⟯⁻¹)->multiply($X->transpose()); return $M[0][0]; }