7 is the only quantity D for which the equation 2n − D = x2 has over two remedies for n and x pure. Especially, the equation 2n − seven = x2 is named the Ramanujan–Nagell equation. Jahannam has seven gates which might be open up constantly. (Qur’an – Sura https://senioraidtools.com/10-easy-mobility-exercises-for-seniors-to-stay-active-and-independent/