Seven is the only real range D for which the equation 2n − D = x2 has a lot more than two remedies for n and x natural. Specifically, the equation 2n − 7 = x2 is known as the Ramanujan–Nagell equation. We're adding a lot more all the time. https://senioraidtools.com/10-easy-mobility-exercises-for-seniors-to-stay-active-and-independent/