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<p>as semimodular lattices.</p>

<p>A semimodular lattice is one kind of <a href="page.php?w=compact_element">algebraic</a> lattice.</p>

<p><big>Birkhoff's condition</big></p>
<p>A lattice is sometimes called <b>weakly semimodular</b> if it satisfies the following condition due to <a href="page.php?w=Garrett_Birkhoff">Garrett Birkhoff</a>:;Birkhoff's condition: If&nbsp;&nbsp; a&nbsp;?&nbsp;b&nbsp;&nbsp;<:&nbsp;&nbsp;a</i> &nbsp;and&nbsp; a&nbsp;?&nbsp;b&nbsp;&nbsp;<:&nbsp;&nbsp;b</i>,<br/>
:then&nbsp;&nbsp; a&nbsp;&nbsp;<:&nbsp;&nbsp;a</i>&nbsp;?&nbsp;b &nbsp;and&nbsp; b&nbsp;&nbsp;<:&nbsp;&nbsp;a</i>&nbsp;?&nbsp;b.Every semimodular lattice is weakly semimodular. The converse is true for lattices of finite length, and more generally for upper continuous (meets distribute over joins of chains) <a href="page.php?w=atom_%28order_theory%29">relatively atomic</a> lattices.</:&nbsp;&nbsp;a</i></:&nbsp;&nbsp;a</i></:&nbsp;&nbsp;b</i></:&nbsp;&nbsp;a</i></p><p>
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