文摘
An invariant subspace M of the Hardy space over the bidisk is said to have two side frames if class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si1.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=f7a12cc6433d5ff78be591283f0f2163" title="Click to view the MathML source">M⊖zMclass="mathContainer hidden">class="mathCode"> and class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si2.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=0b52f00f7597e7fb66fe4914e08a438f" title="Click to view the MathML source">M⊖wMclass="mathContainer hidden">class="mathCode"> contain nonzero class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si3.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=03c61e3dc59ffc79afe1dfba0e2b845e" title="Click to view the MathML source">Twclass="mathContainer hidden">class="mathCode"> and class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si4.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=1ae77992fd520fdb61a82f93a233669f" title="Click to view the MathML source">Tzclass="mathContainer hidden">class="mathCode"> invariant subspaces, respectively. If one frame is in class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si5.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=ae2d615663918db57b8ed9be20e0dbc8" title="Click to view the MathML source">H2(z)class="mathContainer hidden">class="mathCode"> and the other is in class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0022247X15011749&_mathId=si6.gif&_user=111111111&_pii=S0022247X15011749&_rdoc=1&_issn=0022247X&md5=9feba85ed948bc56eb309604a8cc9c09" title="Click to view the MathML source">H2(w)class="mathContainer hidden">class="mathCode">, M is said to have two side rigid frames. We shall show an example of an invariant subspace having two side frames which is not unitarily equivalent to any one having two side rigid frames. We also give some sufficient conditions on M for M to be unitarily equivalent to a rigid one.