seven is the only amount D for which the equation 2n − D = x2 has much more than two answers for n and x pure. Especially, the equation 2n − seven = x2 is known as the Ramanujan–Nagell equation. 2 hrs in the past By Mashable Workforce A MacBook Professional for $280 is unheard of — so seize one just before this deal disappears A MacBook