I need an approximation or a closed form for the series $$ \sum_{a=1}^{M} \frac{1}{a} \, \text{erf} \left(\frac{M}{N} \, a \right) $$
where $M<N$.
I need an approximation or a closed form for the series $$ \sum_{a=1}^{M} \frac{1}{a} \, \text{erf} \left(\frac{M}{N} \, a \right) $$
where $M<N$.
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