public function frobeniusNorm() { $m = $this->m; $n = $this->n; $ΣΣaᵢⱼ² = 0; for ($i = 0; $i < $m; $i++) { for ($j = 0; $j < $n; $j++) { $ΣΣaᵢⱼ² += $this->A[$i][$j] ** 2; } } return sqrt($ΣΣaᵢⱼ²); }