ÖMO 1987 Problem 2

Find the number of all sequences $(x_1,...,x_n)$ of letters $a,b,c$ that satisfy $x_1=x_n=a$ and $x_i \not= x_{i+1}$ for $1 \le i \le n-1.$

Dataset text AI-MO AoPS. Quoted with source.

Solutions

No solution is on olymmath yet. Official and human solutions come first when they arrive. AI solutions come after them, with a label.