Group law on the neutral component of the Jacobian of a real curve of genus 2 having many real components Points rationnels sur \({\mathbb{Q}}\) de la jacobienne de la courbe modulaire \(X_ 0(3^ 4)\). (Rational points over \({\mathbb{Q}}\) of the jacobian of the modular curve \(X_ 0(3^ 4))\) 