7 is the only variety D for which the equation 2n − D = x2 has much more than two options for n and x organic. Specifically, the equation 2n − seven = x2 is called the Ramanujan–Nagell equation. Jahannam has seven gates which are open up always. (Qur’an – https://senioraidtools.com/10-easy-mobility-exercises-for-seniors-to-stay-active-and-independent/