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<b>let </b><font color="Maroon">i0</font> be   <a href="int_1.html#NM1">Integer</a>;<br/>

<b>assume </b><a NAME="E1:29_1"/><i><font color="Green">E23</font></i>: 
<font color="Maroon">F<sub>1</sub></font>() <a href="xxreal_0.html#R1">&lt;=</a> <font color="Maroon">i0</font>
 ;<br/>

<b>defpred </b><font color="Maroon">S<sub>1</sub></font>[ <a href="ordinal1.html#V7">natural</a>  <a href="ordinal1.html#NM1" target="_self">number</a> ] <b>means </b>for <font color="Olive">i2</font> being  <a href="int_1.html#NM1">Integer</a>  st <font color="Maroon">a<sub>1</sub></font> <a href="hidden.html#R1">=</a> <font color="Olive">i2</font> <a href="xcmplx_0.html#K6">-</a> <font color="Maroon">F<sub>1</sub></font>() holds <br/><font color="Maroon">P<sub>1</sub></font>[<font color="Olive">i2</font>];<br/>
<a NAME="E2:29_1"/><i><font color="Green">E24</font></i>: 
<font color="Maroon">S<sub>1</sub></font>[0]
 
<b>by </b><i/>;<br/>
<a NAME="E3:29_1"/><i><font color="Green">E25</font></i>: 
for <font color="Olive">k</font> being   <a href="subset_1.html#M2">Element</a> of  <a href="numbers.html#K5">NAT</a>   st <font color="Maroon">S<sub>1</sub></font>[<font color="Olive">k</font>] holds <br/><font color="Maroon">S<sub>1</sub></font>[<font color="Olive">k</font> <a href="nat_1.html#K1">+</a> 1]
 
<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="29_1_1"><b>proof </b></a><div class="add">

<b>let </b><font color="Maroon">k</font> be    <a href="subset_1.html#M2">Element</a> of  <a href="numbers.html#K5">NAT</a> ;<br/>

<b>reconsider </b><font color="Maroon">i8</font> = <font color="Maroon">k</font> as   <a href="int_1.html#NM1">Integer</a> ;<br/>
<b>assume </b><a NAME="E2:29_1_1"/><i><font color="Green">E31</font></i>: 
<font color="Maroon">S<sub>1</sub></font>[<font color="Maroon">k</font>]
 ;<br/>

<b>let </b><font color="Maroon">i2</font> be   <a href="int_1.html#NM1">Integer</a>;<br/>

<b>assume </b><a NAME="E3:29_1_1"/><i><font color="Green">E32</font></i>: 
<font color="Maroon">k</font> <a href="nat_1.html#K1">+</a> 1 <a href="hidden.html#R1">=</a> <font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> <font color="Maroon">F<sub>1</sub></font>()
 ;<br/>

<a NAME="E4:29_1_1"/><b>then </b>
<font color="Maroon">k</font> <a href="hidden.html#R1">=</a> <span class="p1">(<span class="default"><font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1</span>)</span> <a href="xcmplx_0.html#K6">-</a> <font color="Maroon">F<sub>1</sub></font>()
 
;<br/>
<a NAME="E5:29_1_1"/><b>then </b><i><font color="Green">E33</font></i>: 
<font color="Maroon">P<sub>1</sub></font>[<font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1]
 
<b>by </b><i><a class="txt" href="int_1.html#E2:29_1"><i><font color="Green">E24</font></i></a></i>;<br/>
<a NAME="E6:29_1_1"/>
<font color="Maroon">F<sub>1</sub></font>() <a href="xxreal_0.html#R1">&lt;=</a> <font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1
 
<div><a class="txt" onclick="hs2(this)" href="javascript:()" title="29_1_1_1"><b>proof </b></a><div class="add">

<a NAME="E1:29_1_1_1"/>
<font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1 <a href="hidden.html#R1">=</a> <font color="Maroon">i8</font> <a href="xcmplx_0.html#K2">+</a> <font color="Maroon">F<sub>1</sub></font>()
 
<b>by </b><i><a class="txt" href="int_1.html#E3:29_1"><i><font color="Green">E25</font></i></a></i>;<br/>
<b>hence </b><a NAME="E2:29_1_1_1"/>
<font color="Maroon">F<sub>1</sub></font>() <a href="xxreal_0.html#R1">&lt;=</a> <font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1
 <b>by </b><i/>;<br/>


</div><b>end;</b></div>
<a NAME="E7:29_1_1"/><b>then </b>
<font color="Maroon">P<sub>1</sub></font>[<span class="p1">(<span class="default"><font color="Maroon">i2</font> <a href="xcmplx_0.html#K6">-</a> 1</span>)</span> <a href="xcmplx_0.html#K2">+</a> 1]
 
<b>by </b><i><a class="txt" href="int_1.html#E1"><i><font color="Green">Lemma20</font></i></a>, </i>;<br/>
<b>hence </b><a NAME="E8:29_1_1"/>
<font color="Maroon">P<sub>1</sub></font>[<font color="Maroon">i2</font>]
 ;<br/>


</div><b>end;</b></div>
<a NAME="E4:29_1"/><i><font color="Green">E34</font></i>: 
for <font color="Olive">k</font> being   <a href="subset_1.html#M2">Element</a> of  <a href="numbers.html#K5">NAT</a>  holds  <font color="Maroon">S<sub>1</sub></font>[<font color="Olive">k</font>]
 
<b>from </b><i><a class="ref" href="nat_1.html#S1">NAT_1:sch 1</a>(<a class="txt" href="int_1.html#E2:29"><i><font color="Green">E22</font></i></a>, <a class="txt" href="int_1.html#E1:29_1"><i><font color="Green">E23</font></i></a>);<br/></i>
<b>reconsider </b><font color="Maroon">l</font> = <font color="Maroon">i0</font> <a href="xcmplx_0.html#K6">-</a> <font color="Maroon">F<sub>1</sub></font>() as    <a href="subset_1.html#M2">Element</a> of  <a href="numbers.html#K5">NAT</a>  <b>by </b><i><a class="txt" href="int_1.html#E1:29"><i><font color="Green">E21</font></i></a>, </i>;<br/>
<a NAME="E6:29_1"/>
<font color="Maroon">l</font> <a href="hidden.html#R1">=</a> <font color="Maroon">i0</font> <a href="xcmplx_0.html#K6">-</a> <font color="Maroon">F<sub>1</sub></font>()
 
;<br/>
<b>hence </b><a NAME="E7:29_1"/>
<font color="Maroon">P<sub>1</sub></font>[<font color="Maroon">i0</font>]
 <b>by </b><i><a class="txt" href="int_1.html#E2:29_1"><i><font color="Green">E24</font></i></a></i>;<br/>


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