f(x) = 1 + sum k=1 to infinity(9/(k^2*Pi^2)*cos(2/(3*k*Pi)*x)-3/(k*Pi)*sin(2/(3*k*Pi)*x))f(x) = 1 + sum k=1 to infinity(9/(k^2*Pi^2)*cos(2/(3*k*Pi)*x)-3/(k*Pi)*sin(2/(3*k*Pi)*x))

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