213585706927domain: NAppears in sequencesa(n) = a(1)*a(n-1) + a(2)*a(n-2) + ... + a(n-3)*a(3) for n >= 4, with initial terms 3, 1, 1, 3.at n=22A025254Diagonal sums of second binomial transform of the Narayana triangle A001263.at n=20A178578