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<b>let </b><font color="Maroon">c<sub>1</sub></font> be    <a href="hidden.html#M1">set</a> ;<br/><b>let </b><font color="Maroon">c<sub>2</sub></font> be   <a href="setfam_1.html#NM1">Subset-Family</a> of <font color="Maroon">c<sub>1</sub></font>;<br/>



<a NAME="E1:2"/>
<font color="Maroon">c<sub>2</sub></font>, <a href="setfam_1.html#K7">COMPLEMENT</a> <font color="Maroon">c<sub>2</sub></font> <a href="tarski.html#R2">are_equipotent</a> 
 
<b>by </b><i><a class="ref" href="yellow_8.html#T4">YELLOW_8:4</a></i>;<br/>
<b>hence </b><a NAME="E2:2"/>
( <font color="Maroon">c<sub>2</sub></font> is <a href="card_4.html#V1">countable</a> iff  <a href="setfam_1.html#K7">COMPLEMENT</a> <font color="Maroon">c<sub>2</sub></font> is <a href="card_4.html#V1">countable</a> )
 <b>by </b><i><a class="ref" href="yellow_8.html#T5">YELLOW_8:5</a></i>;<br/>


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