 defined in [<a href="doc_spams010.html#friedman">10</a>] with the homotopy method presented in the appendix of [<a href="doc_spams010.html#mairal9">21</a>];
</li><li class="li-itemize">a tool for projecting efficiently onto a few convex sets
inducing sparsity such as the ℓ<sub>1</sub>-ball using the method of
[<a href="doc_spams010.html#brucker">3</a>, <a href="doc_spams010.html#maculan">18</a>, <a href="doc_spams010.html#duchi">8</a>], and Elastic-Net or Fused Lasso constraint sets as
proposed in the 