1527506domain: NAppears in sequencesa(n) is the number of nonnegative integers k less than 10^n such that the decimal representation of k lacks digits 1 and 2, at least one of digits 3,4,5 and at least one of digits 6,7,8,9.at n=6A126629a(n) = (n!)^2 * Sum_{k=0..n-1} 1 / ((n-k) * k!)^2.at n=7A346410