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<b>thus </b><a NAME="E1:66"/>
 <a href="card_1.html#K3" target="_self">alef</a> 0 <a href="ordinal1.html#R1">c=</a>  <a href="ordinal1.html#K5">omega</a> 
 <b>by </b><i>, </i>; <i><font color="Red">:: according to </font></i><a class="ref" href="xboole_0.html#D10">XBOOLE_0:def 10</a><br/>

<b>thus </b><a NAME="E2:66"/>
 <a href="ordinal1.html#K5">omega</a>  <a href="ordinal1.html#R1">c=</a>  <a href="card_1.html#K3" target="_self">alef</a> 0
 <div><a class="txt" onclick="hs2(this)" href="javascript:()" title="66_1"><b>proof </b></a><div class="add">

<b>let </b><font color="Maroon">x</font> be    <a href="hidden.html#M1">set</a> ; <i><font color="Red">:: according to </font></i><a class="ref" href="tarski.html#D3">TARSKI:def 3</a><br/>

<b>assume </b><a NAME="E1:66_1"/><i><font color="Green">E33</font></i>: 
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="ordinal1.html#K5">omega</a> 
 ;<br/>

<b>then reconsider </b><font color="Maroon">A</font> = <font color="Maroon">x</font> as   <a href="ordinal1.html#NM2">Ordinal</a> <b>by </b><i><a class="ref" href="ordinal1.html#T23">ORDINAL1:23</a></i>;<br/>
<b>consider </b><font color="Maroon">n</font> being    <a href="subset_1.html#M1">Element</a> of  <a href="numbers.html#K5">NAT</a> <b> such that </b><br/><a NAME="E4:66_1"/><i><font color="Green">E34</font></i>: 
<font color="Maroon">A</font> <a href="hidden.html#R1">=</a> <font color="Maroon">n</font>
 <b>by </b><i/>;<br/>
<a NAME="E5:66_1"/><i><font color="Green">E42</font></i>: 
(  <a href="ordinal1.html#K1">succ</a> <font color="Maroon">n</font> <a href="ordinal1.html#R1">c=</a>  <a href="ordinal1.html#K5">omega</a>  &amp; <font color="Maroon">n</font> <a href="nat_1.html#K1">+</a> 1 <a href="hidden.html#R1">=</a>  <a href="ordinal1.html#K1">succ</a> <font color="Maroon">n</font> )
 
<b>by </b><i>, <a class="ref" href="ordinal1.html#T33">ORDINAL1:33</a></i>;<br/>
<a NAME="E6:66_1"/><b>then </b>
(  <a href="card_1.html#K1" target="_self">Card</a> <span class="p1">(<span class="default"><font color="Maroon">n</font> <a href="nat_1.html#K1">+</a> 1</span>)</span> <a href="ordinal1.html#R1">c=</a>  <a href="card_1.html#K1" target="_self">Card</a> <a href="ordinal1.html#K5">omega</a>  &amp; <font color="Maroon">n</font> <a href="hidden.html#R2">in</a>  <a href="ordinal1.html#K1">succ</a> <font color="Maroon">n</font> &amp;  <a href="card_1.html#K1" target="_self">Card</a> <span class="p1">(<span class="default"><font color="Maroon">n</font> <a href="nat_1.html#K1">+</a> 1</span>)</span> <a href="hidden.html#R1">=</a> <font color="Maroon">n</font> <a href="nat_1.html#K1">+</a> 1 )
 
<b>by </b><i>, , <a class="ref" href="ordinal1.html#T10">ORDINAL1:10</a></i>;<br/>
<b>hence </b><a NAME="E7:66_1"/>
<font color="Maroon">x</font> <a href="hidden.html#R2">in</a>  <a href="card_1.html#K3" target="_self">alef</a> 0
 <b>by </b><i><a class="ref" href="card_1.html#T8" target="_self">Th8</a>, <a class="ref" href="card_1.html#D5" target="_self">Def5</a>, </i>;<br/>


</div><b>end;</b></div>


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