The trace norm of the digraph D is defined as class="mathmlsrc">title="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516301525&_mathId=si1.gif&_user=111111111&_pii=S0024379516301525&_rdoc=1&_issn=00243795&md5=0992e8c20e6ae11de6ed856ea0440c6a">class="imgLazyJSB inlineImage" height="35" width="106" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516301525-si1.gif">class="mathContainer hidden">class="mathCode">, where class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516301525&_mathId=si2.gif&_user=111111111&_pii=S0024379516301525&_rdoc=1&_issn=00243795&md5=2e922844319182f746c7648b6429240c" title="Click to view the MathML source">σ1≥σ2≥⋯≥σn≥0class="mathContainer hidden">class="mathCode"> are the singular values of the adjacency matrix A of D , i.e. the square roots of the eigenvalues of class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516301525&_mathId=si3.gif&_user=111111111&_pii=S0024379516301525&_rdoc=1&_issn=00243795&md5=7e491f2dc8f31c22fb521cbdff1be93b" title="Click to view the MathML source">AA⊤class="mathContainer hidden">class="mathCode">. We find the extremal values of class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516301525&_mathId=si14.gif&_user=111111111&_pii=S0024379516301525&_rdoc=1&_issn=00243795&md5=570acf07d121022de55161dfde57f4e8" title="Click to view the MathML source">Nclass="mathContainer hidden">class="mathCode"> over the set of oriented trees and over the set of oriented paths.