G.f. (2-x-2*x^2-x^3)/(x^4+2*x^3-x^2-2*x+1). 2, 3, 6, 10, 18, 31, 54, 93, 160, 274, 468, 797, 1354, 2295, 3882, 6554, 11046, 18587
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