a(n) = 2*a(n-1)^2 - 1, a(0) = 4, a(1) = 31.A005828a(n) = 2*a(n-1)^2 - 1, a(0) = 4, a(1) = 31.Terms a(0) =4a(1) =31a(2) =1921a(3) =7380481External referencesoeis: A005828