public static function square(array $xs) : array { return array_map(function ($x) { return $x ** 2; }, $xs); }
/** * Sum of squares * * ∑⟮xᵢ⟯² * * @param array $numbers * * @return number */ public static function sumOfSquares(array $numbers) { if (empty($numbers)) { return null; } $∑⟮xᵢ⟯² = array_sum(Map\Single::square($numbers)); return $∑⟮xᵢ⟯²; }