Sparsemax is a type of activation/output function similar to the traditional softmax, but able to output sparse probabilities.
sparsemax(z)=arg_p∈ΔK−1min∣∣p−z∣∣2\text{sparsemax}\left(z\right) = \arg\_{p∈\Delta^{K−1}}\min||\mathbf{p} - \mathbf{z}||^{2}sparsemax(z)=arg_p∈ΔK−1min∣∣p−z∣∣2