a(n) = (2^(n^2) - 1)/(2^n - 1).A128889a(n) = (2^(n^2) - 1)/(2^n - 1).Terms a(0) =1a(1) =5a(2) =73a(3) =4369a(4) =1082401a(5) =1090785345External referencesoeis: A128889