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Another solution is to solve $x^n=x^(n-1)+2*x^(n-2)$. The roots of the equation are -1, 0 and 2 that means the solution can be found in the form $a_n = c*(-1)^n+d*0^n+e*2^n$. Using a_1 = 1, a_2 = 8 and 0^n=0 it is easy to find koefficients