61732716domain: NAppears in sequencesSum_{i=1..(10^n - 1)/9} i, or ((10^n -1)/9)*((10^n -1)/9 +1)/2 (n-th term is the middle 2(n-1) digits of the (n+9)-th term for n > 1).at n=4A003555Duplicate of A003555.at n=4A099639