45813071872domain: NAppears in sequencesa(n) = 2^(n-1)*(2^n - (-1)^n)/3.at n=19A003683Starting a priori with the fraction 1/1, the denominators of fractions built according to the rule: add top and bottom to get the new bottom, add top and 9 times the bottom to get the new top.at n=17A110953(A192466)/2.at n=18A192467