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<b>let </b><font color="Maroon" title="c2">X0</font> be  non <a href="struct_0.html#V2" title="STRUCT_0:attr.2">empty</a>  <a href="pre_topc.html#M1" title="PRE_TOPC:mode.1">SubSpace</a> of <font color="Maroon" title="c1">X</font>;<br/>

<b>reconsider </b><font color="Maroon" title="c3">A</font> = the <a href="struct_0.html#U1" title="STRUCT_0:sel.1">carrier</a> of <font color="Maroon" title="c2">X0</font> as  non <a href="xboole_0.html#V1" title="XBOOLE_0:attr.1">empty</a> <a href="struct_0.html#NM2" title="STRUCT_0:NM.2">Subset</a> of <font color="Maroon" title="c1">X</font> <b>by </b><i><a class="ref" href="tsep_1.html#T1" title="TSEP_1:th.1">TSEP_1:1</a></i>;<br/>
<b>assume </b><a NAME="E2:73_4_1"/><i><font color="Green" title="E27">A2</font></i>: 
<font color="Maroon" title="c2">X0</font> is <a href="tex_3.html#V3" title="TEX_3:attr.3">boundary</a>
 ;<br/>

<b>assume </b><a NAME="E3:73_4_1"/><i><font color="Green" title="E28">A3</font></i>: 
<font color="Maroon" title="c2">X0</font> is <a href="borsuk_1.html#V1" title="BORSUK_1:attr.1">closed</a>
 ;<br/>

<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="73_4_1_1"><b>now </b></a><div class="add"><b>take </b><font color="Maroon" title="c4">A</font> = <font color="Maroon" title="c3">A</font>;<br/><b>thus </b><a NAME="E1:73_4_1_1"/>
( <font color="Maroon" title="c4">A</font> is <a href="tops_1.html#V2" title="TOPS_1:attr.2">boundary</a> &amp; <font color="Maroon" title="c4">A</font> is <a href="pre_topc.html#V4" title="PRE_TOPC:attr.4">closed</a> )
 <b>by </b><i><a class="txt" href="tex_3.html#E2:73_4_1"><i><font color="Green" title="E27">A2</font></i></a>, <a class="txt" href="tex_3.html#E3:73_4_1"><i><font color="Green" title="E28">A3</font></i></a>, <a class="ref" href="tex_3.html#T29" target="_self" title="TEX_3:th.29">Th29</a>, <a class="ref" href="tsep_1.html#T11" title="TSEP_1:th.11">TSEP_1:11</a></i>;<br/></div><b>end;</b></div>
<b>hence </b><a NAME="E5:73_4_1"/>
contradiction
 <b>by </b><i><a class="ref" href="tex_1.html#T37" title="TEX_1:th.37">TEX_1:37</a></i>;<br/>


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