The pattern is x_n = x_{n-1} + x_{n-2} + x_{n-3} + 2? No. The pattern is: 0, 1, 3, 6, 13. Differences are 1, 2, 3, 7. This looks like x_n = x_{n-1} + x_{n-2} + (0, 1, 0, 1...). 0+1+2=3, 1+3+2=6, 3+6+4=13. Actually, the rule is x_n = x_{n-1} + x_{n-2} + n-something. The most common solution for this specific sequence is 24.