a(n) = a(n-1) + (-3)^(n-1) = (1 - (-3)^n)/4. G.f.: x/((1-x)*(1+3*x)). 0, 1, 2, 7, 20, 61, 182, 547, 1640, 4921, 14762, 44287, 132860, 398581
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