3131313131domain: NAppears in sequencesDecimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 3,1.at n=9A037589Concatenation of segments of the digit sequence 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3....at n=9A137318