800589domain: NAppears in sequencesa(1) = 4; a(n) = smallest composite number of form k*a(n-1) + 1 with k > 1.at n=15A071258Positive integers m such that 2*m^2 - 1 = x^4 + y^4 for some nonnegative integers x and y with |x - y| > 1.at n=19A343913