For two
n-by-
n matrices
A and
B , it was known before that their numerica
l radii satisfy the inequa
lity
lsi1" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si1.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=61a01a813ddd5dd90e6d05f03fe0bb44" title="Click to view the MathML source">w(AB)≤4w(A)w(B)lass="mathContainer hidden">lass="mathCode">, and the equa
lity is attained by the 2-by-2 matrices
lsi2" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si2.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=d20ee60949ff7061a5e52ac978e5c60c">lass="imgLazyJSB inlineImage" height="21" width="66" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si2.gif">lass="mathContainer hidden">lass="mathCode"> and
lsi3" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si3.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=9ebdfe37d20ebdd73bdd5388ec7aac79">lass="imgLazyJSB inlineImage" height="21" width="67" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si3.gif">lass="mathContainer hidden">lass="mathCode">. Moreover, the constant &
ldquo;4” here can be reduced to &
ldquo;2” if
A and
B commute, and the corresponding equa
lity is attained by
lsi4" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si4.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=55ea59825a8be093ee3c3cb00b1714a4">lass="imgLazyJSB inlineImage" height="21" width="103" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si4.gif">lass="mathContainer hidden">lass="mathCode"> and
lsi5" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si5.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=3f1fa01217b032ae75607c3ae63a1132">lass="imgLazyJSB inlineImage" height="21" width="106" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si5.gif">lass="mathContainer hidden">lass="mathCode">. In this paper, we give a comp
lete characterization of
A and
B for which the equa
lity ho
lds in each case. More precise
ly, it is shown that
lsi57" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si57.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=bde2959537683c3517043bf45f3c0c1f" title="Click to view the MathML source">w(AB)=4w(A)w(B)lass="mathContainer hidden">lass="mathCode"> (resp.,
lsi159" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si159.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=4ecdc2a702fe23c512872e6aad6d4c46" title="Click to view the MathML source">w(AB)=2w(A)w(B)lass="mathContainer hidden">lass="mathCode"> for commuting
A and
B) if and on
ly if either
A or
B is the zero matrix, or
A and
B are simu
ltaneous
ly unitari
ly simi
lar to matrices of the form
lsi26" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si26.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=a7093c9bf96862e757ad398658340dad">lass="imgLazyJSB inlineImage" height="21" width="71" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si26.gif">lass="mathContainer hidden">lass="mathCode"> and
lsi27" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si27.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=eb3d18a08cef7698a303c3b92b58e119">lass="imgLazyJSB inlineImage" height="21" width="72" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si27.gif">lass="mathContainer hidden">lass="mathCode"> (resp.,
lass="formu
la" id="fm0010">
with
lsi11" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si11.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=1e229d0f94852204a933e39eec5bc835" title="Click to view the MathML source">w(A′)≤|a|/2lass="mathContainer hidden">lass="mathCode"> and
lsi12" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si12.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=164ad8b64e6d682a98ac701b0d6e19d5" title="Click to view the MathML source">w(B′)≤|b|/2lass="mathContainer hidden">lass="mathCode">. An ana
logous characterization for the extrema
l equa
lity for tensor products is a
lso proven. For doub
ly commuting matrices, we use their unitary simi
larity mode
l to obtain the corresponding resu
lt. For commuting 2-by-2 matrices
A and
B , we show that
lsi13" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si13.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=f97c4b18274f69cb2bc0035322ca86a1" title="Click to view the MathML source">w(AB)=w(A)w(B)lass="mathContainer hidden">lass="mathCode"> if and on
ly if either
A or
B is a sca
lar matrix, or
A and
B are simu
ltaneous
ly unitari
ly simi
lar to
lsi14" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si14.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=14ace1731c9857477311226c5cbe8450">lass="imgLazyJSB inlineImage" height="29" width="46" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si14.gif">lass="mathContainer hidden">lass="mathCode"> and
lsi15" class="mathmlsrc">le="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si15.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=27e83bfbf0dc5c7b801a2b4d06375b28">lass="imgLazyJSB inlineImage" height="29" width="44" alt="View the MathML source" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S0024379516300076-si15.gif">lass="mathContainer hidden">lass="mathCode"> with
lsi16" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si16.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=c2023d12b941a19a61824e6063cc325f" title="Click to view the MathML source">|a1|≥|a2|lass="mathContainer hidden">lass="mathCode"> and
lsi17" class="mathmlsrc">lass="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0024379516300076&_mathId=si17.gif&_user=111111111&_pii=S0024379516300076&_rdoc=1&_issn=00243795&md5=e89e65508fd5889c24ebe41a8ad6aab3" title="Click to view the MathML source">|b1|≥|b2|lass="mathContainer hidden">lass="mathCode">.