@article{JEMS_2017_19_6_a5,
     author = {J. William Helton and Igor Klep and Scott McCullough},
     title = {The tracial {Hahn{\textendash}Banach} theorem, polar duals, matrix convex sets, and projections of free spectrahedra},
     journal = {Journal of the European Mathematical Society},
     pages = {1845--1897},
     publisher = {mathdoc},
     volume = {19},
     number = {6},
     year = {2017},
     doi = {10.4171/jems/707},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/707/}
}