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Good idea!
So with last beta http://beta.plantuml.net/plantuml.jar you can now have:
@startmath scale 5 color red f(t)=(a_0)/2 + sum_(n=1)^ooa_ncos((npit)/L)+sum_(n=1)^oo b_n\ sin((npit)/L) @endmath
Is this what you are expecting ?