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== Axiomatics == {| align="right" class="wikitable collapsible collapsed" style="text-align:left" ! colspan="2" | '''Proven properties''' |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''UId<sub>1</sub>''' !! !! colspan="2" | If ''x'' ∨ ''o'' = ''x'' for all ''x'', then ''o'' = 0 |- | Proof: || || colspan="2" | If ''x'' ∨ ''o'' = ''x'', then |- | || || 0 |- | || = || 0 ∨ ''o'' || by assumption |- | || = || ''o'' ∨ 0 || by '''Cmm<sub>1</sub>''' |- | || = || ''o'' || by '''Idn<sub>1</sub>''' |} | '''UId<sub>2</sub>''' [dual] If ''x'' ∧ ''i'' = ''x'' for all ''x'', then ''i'' = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''Idm<sub>1</sub>''' !! !! ''x'' ∨ ''x'' = ''x'' |- | Proof: || || ''x'' ∨ ''x'' |- | || = || (''x'' ∨ ''x'') ∧ 1 || by '''Idn<sub>2</sub>''' |- | || = || (''x'' ∨ ''x'') ∧ (''x'' ∨ ¬''x'') || by '''Cpl<sub>1</sub>''' |- | || = || ''x'' ∨ (''x'' ∧ ¬''x'') || by '''Dst<sub>1</sub>''' |- | || = || ''x'' ∨ 0 || by '''Cpl<sub>2</sub>''' |- | || = || ''x'' || by '''Idn<sub>1</sub>''' |} | '''Idm<sub>2</sub>''' [dual] ''x'' ∧ ''x'' = ''x'' |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''Bnd<sub>1</sub>''' !! !! ''x'' ∨ 1 = 1 |- | Proof: || || ''x'' ∨ 1 |- | || = || (''x'' ∨ 1) ∧ 1 || by '''Idn<sub>2</sub>''' |- | || = || 1 ∧ (''x'' ∨ 1) || by '''Cmm<sub>2</sub>''' |- | || = || (''x'' ∨ ¬''x'') ∧ (''x'' ∨ 1) || by '''Cpl<sub>1</sub>''' |- | || = || ''x'' ∨ (¬''x'' ∧ 1) || by '''Dst<sub>1</sub>''' |- | || = || ''x'' ∨ ¬''x'' || by '''Idn<sub>2</sub>''' |- | || = || 1 || by '''Cpl<sub>1</sub>''' |} | '''Bnd<sub>2</sub>''' [dual] ''x'' ∧ 0 = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''Abs<sub>1</sub>''' !! !! ''x'' ∨ (''x'' ∧ ''y'') = ''x'' |- | Proof: || || ''x'' ∨ (''x'' ∧ ''y'') |- | || = || (''x'' ∧ 1) ∨ (''x'' ∧ ''y'') || by '''Idn<sub>2</sub>''' |- | || = || ''x'' ∧ (1 ∨ ''y'') || by '''Dst<sub>2</sub>''' |- | || = || ''x'' ∧ (''y'' ∨ 1) || by '''Cmm<sub>1</sub>''' |- | || = || ''x'' ∧ 1 || by '''Bnd<sub>1</sub>''' |- | || = || ''x'' || by '''Idn<sub>2</sub>''' |} | '''Abs<sub>2</sub>''' [dual] ''x'' ∧ (''x'' ∨ ''y'') = ''x'' |- valign="top" | colspan="2" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''UNg''' !! !! colspan="2" | If ''x'' ∨ ''x''<sub>n</sub> = 1 and ''x'' ∧ ''x''<sub>n</sub> = 0, then ''x''<sub>n</sub> = ¬''x'' |- | Proof: || || colspan="2" | If ''x'' ∨ ''x''<sub>n</sub> = 1 and ''x'' ∧ ''x''<sub>n</sub> = 0, then |- | || ||''x''<sub>n</sub> |- | || = || ''x''<sub>n</sub> ∧ 1 || by '''Idn<sub>2</sub>''' |- | || = || ''x''<sub>n</sub> ∧ (''x'' ∨ ¬''x'') || by '''Cpl<sub>1</sub>''' |- | || = || (''x''<sub>n</sub> ∧ ''x'') ∨ (''x''<sub>n</sub> ∧ ¬''x'') || by '''Dst<sub>2</sub>''' |- | || = || (''x'' ∧ ''x''<sub>n</sub>) ∨ (¬''x'' ∧ ''x''<sub>n</sub>) || by '''Cmm<sub>2</sub>''' |- | || = || 0 ∨ (¬''x'' ∧ ''x''<sub>n</sub>) || by assumption |- | || = || (''x'' ∧ ¬''x'') ∨ (¬''x'' ∧ ''x''<sub>n</sub>) || by '''Cpl<sub>2</sub>''' |- | || = || (¬''x'' ∧ ''x'') ∨ (¬''x'' ∧ ''x''<sub>n</sub>) || by '''Cmm<sub>2</sub>''' |- | || = || ¬''x'' ∧ (''x'' ∨ ''x''<sub>n</sub>) || by '''Dst<sub>2</sub>''' |- | || = || ¬''x'' ∧ 1 || by assumption |- | || = || ¬''x'' || by '''Idn<sub>2</sub>''' |} |- valign="top" | colspan="2" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''DNg''' !! !! ¬¬''x'' = ''x'' |- | Proof: || || ¬''x'' ∨ ''x'' = ''x'' ∨ ¬''x'' = 1 || by '''Cmm<sub>1</sub>''', '''Cpl<sub>1</sub>''' |- | || and || ¬''x'' ∧ ''x'' = ''x'' ∧ ¬''x'' = 0 || by '''Cmm<sub>2</sub>''', '''Cpl<sub>2</sub>''' |- | || hence || ''x'' = ¬¬''x'' || by '''UNg''' |} |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''A<sub>1</sub>''' !! !! ''x'' ∨ (¬''x'' ∨ ''y'') = 1 |- | Proof: || || ''x'' ∨ (¬''x'' ∨ ''y'') |- | || = || (''x'' ∨ (¬''x'' ∨ ''y'')) ∧ 1 || by '''Idn<sub>2</sub>''' |- | || = || 1 ∧ (''x'' ∨ (¬''x'' ∨ ''y'')) || by '''Cmm<sub>2</sub>''' |- | || = || (''x'' ∨ ¬''x'') ∧ (''x'' ∨ (¬''x'' ∨ ''y'')) || by '''Cpl<sub>1</sub>''' |- | || = || ''x'' ∨ (¬''x'' ∧ (¬''x'' ∨ ''y'')) || by '''Dst<sub>1</sub>''' |- | || = || ''x'' ∨ ¬''x'' || by '''Abs<sub>2</sub>''' |- | || = || 1 || by '''Cpl<sub>1</sub>''' |} | '''A<sub>2</sub>''' [dual] ''x'' ∧ (¬''x'' ∧ ''y'') = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''B<sub>1</sub>''' !! !! (''x'' ∨ ''y'') ∨ (¬''x'' ∧ ¬''y'') = 1 |- | Proof: || || (''x'' ∨ ''y'') ∨ (¬''x'' ∧ ¬''y'') |- | || = || ((''x'' ∨ ''y'') ∨ ¬''x'') ∧ ((''x'' ∨ ''y'') ∨ ¬''y'') || by '''Dst<sub>1</sub>''' |- | || = || (¬''x'' ∨ (''x'' ∨ ''y'')) ∧ (¬''y'' ∨ (''y'' ∨ ''x'')) || by '''Cmm<sub>1</sub>''' |- | || = || (¬''x'' ∨ (¬¬''x'' ∨ ''y'')) ∧ (¬''y'' ∨ (¬¬''y'' ∨ ''x'')) || by '''DNg''' |- | || = || 1 ∧ 1 || by '''A<sub>1</sub>''' |- | || = || 1 || by '''Idn<sub>2</sub>''' |} | '''B<sub>2</sub>''' [dual] (''x'' ∧ ''y'') ∧ (¬''x'' ∨ ¬''y'') = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''C<sub>1</sub>''' !! !! (''x'' ∨ ''y'') ∧ (¬''x'' ∧ ¬''y'') = 0 |- | Proof: || || (''x'' ∨ ''y'') ∧ (¬''x'' ∧ ¬''y'') |- | || = || (¬''x'' ∧ ¬''y'') ∧ (''x'' ∨ ''y'') || by '''Cmm<sub>2</sub>''' |- | || = || ((¬''x'' ∧ ¬''y'') ∧ ''x'') ∨ ((¬''x'' ∧ ¬''y'') ∧ ''y'') || by '''Dst<sub>2</sub>''' |- | || = || (''x'' ∧ (¬''x'' ∧ ¬''y'')) ∨ (''y'' ∧ (¬''y'' ∧ ¬''x'')) || by '''Cmm<sub>2</sub>''' |- | || = || 0 ∨ 0 || by '''A<sub>2</sub>''' |- | || = || 0 || by '''Idn<sub>1</sub>''' |} | '''C<sub>2</sub>''' [dual] (''x'' ∧ ''y'') ∨ (¬''x'' ∨ ¬''y'') = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''DMg<sub>1</sub>''' !! !! ¬(''x'' ∨ ''y'') = ¬''x'' ∧ ¬''y'' |- | Proof: || || by '''B<sub>1</sub>''', '''C<sub>1</sub>''', and '''UNg''' |} | '''DMg<sub>2</sub>''' [dual] ¬(''x'' ∧ ''y'') = ¬''x'' ∨ ¬''y'' |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''D<sub>1</sub>''' !! !! (''x''∨(''y''∨''z'')) ∨ ¬''x'' = 1 |- | Proof: || || (''x'' ∨ (''y'' ∨ ''z'')) ∨ ¬''x'' |- | || = || ¬''x'' ∨ (''x'' ∨ (''y'' ∨ ''z'')) || by '''Cmm<sub>1</sub>''' |- | || = || ¬''x'' ∨ (¬¬''x'' ∨ (''y'' ∨ ''z'')) || by '''DNg''' |- | || = || 1 || by '''A<sub>1</sub>''' |} | '''D<sub>2</sub>''' [dual] (''x''∧(''y''∧''z'')) ∧ ¬''x'' = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''E<sub>1</sub>''' !! !! ''y'' ∧ (''x''∨(''y''∨''z'')) = ''y'' |- | Proof: || || ''y'' ∧ (''x'' ∨ (''y'' ∨ ''z'')) |- | || = || (''y'' ∧ ''x'') ∨ (''y'' ∧ (''y'' ∨ ''z'')) || by '''Dst<sub>2</sub>''' |- | || = || (''y'' ∧ ''x'') ∨ ''y'' || by '''Abs<sub>2</sub>''' |- | || = || ''y'' ∨ (''y'' ∧ ''x'') || by '''Cmm<sub>1</sub>''' |- | || = || ''y'' || by '''Abs<sub>1</sub>''' |} | '''E<sub>2</sub>''' [dual] ''y'' ∨ (''x''∧(''y''∧''z'')) = ''y'' |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''F<sub>1</sub>''' !! !! (''x''∨(''y''∨''z'')) ∨ ¬''y'' = 1 |- | Proof: || || (''x'' ∨ (''y'' ∨ ''z'')) ∨ ¬''y'' |- | || = || ¬''y'' ∨ (''x'' ∨ (''y'' ∨ ''z'')) || by '''Cmm<sub>1</sub>''' |- | || = || (¬''y'' ∨ (''x'' ∨ (''y'' ∨ ''z''))) ∧ 1 || by '''Idn<sub>2</sub>''' |- | || = || 1 ∧ (¬''y'' ∨ (''x'' ∨ (''y'' ∨ ''z''))) || by '''Cmm<sub>2</sub>''' |- | || = || (''y'' ∨ ¬''y'') ∧ (¬''y'' ∨ (''x'' ∨ (''y'' ∨ ''z''))) || by '''Cpl<sub>1</sub>''' |- | || = || (¬''y'' ∨ ''y'') ∧ (¬''y'' ∨ (''x'' ∨ (''y'' ∨ ''z''))) || by '''Cmm<sub>1</sub>''' |- | || = || ¬''y'' ∨ (''y'' ∧ (''x'' ∨ (''y'' ∨ ''z''))) || by '''Dst<sub>1</sub>''' |- | || = || ¬''y'' ∨ ''y'' || by '''E<sub>1</sub>''' |- | || = || ''y'' ∨ ¬''y'' || by '''Cmm<sub>1</sub>''' |- | || = || 1 || by '''Cpl<sub>1</sub>''' |} | '''F<sub>2</sub>''' [dual] (''x''∧(''y''∧''z'')) ∧ ¬''y'' = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''G<sub>1</sub>''' !! !! (''x''∨(''y''∨''z'')) ∨ ¬''z'' = 1 |- | Proof: || || (''x'' ∨ (''y'' ∨ ''z'')) ∨ ¬''z'' |- | || = || (''x'' ∨ (''z'' ∨ ''y'')) ∨ ¬''z'' || by '''Cmm<sub>1</sub>''' |- | || = || 1 || by '''F<sub>1</sub>''' |} | '''G<sub>2</sub>''' [dual] (''x''∧(''y''∧''z'')) ∧ ¬''z'' = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''H<sub>1</sub>''' !! !! ¬((''x''∨''y'')∨''z'') ∧ ''x'' = 0 |- | Proof: || || ¬((''x'' ∨ ''y'') ∨ ''z'') ∧ ''x'' |- | || = || (¬(''x'' ∨ ''y'') ∧ ¬''z'') ∧ ''x'' || by '''DMg<sub>1</sub>''' |- | || = || ((¬''x'' ∧ ¬''y'') ∧ ¬''z'') ∧ ''x'' || by '''DMg<sub>1</sub>''' |- | || = || ''x'' ∧ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'') || by '''Cmm<sub>2</sub>''' |- | || = || (''x'' ∧ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'')) ∨ 0 || by '''Idn<sub>1</sub>''' |- | || = || 0 ∨ (''x'' ∧ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'')) || by '''Cmm<sub>1</sub>''' |- | || = || (''x'' ∧ ¬''x'') ∨ (''x'' ∧ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'')) || by '''Cpl<sub>2</sub>''' |- | || = || ''x'' ∧ (¬''x'' ∨ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'')) || by '''Dst<sub>2</sub>''' |- | || = || ''x'' ∧ (¬''x'' ∨ (¬''z'' ∧ (¬''x'' ∧ ¬''y''))) || by '''Cmm<sub>2</sub>''' |- | || = || ''x'' ∧ ¬''x'' || by '''E<sub>2</sub>''' |- | || = || 0 || by '''Cpl<sub>2</sub>''' |} | '''H<sub>2</sub>''' [dual] ¬((''x''∧''y'')∧''z'') ∨ ''x'' = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''I<sub>1</sub>''' !! !! ¬((''x''∨''y'')∨''z'') ∧ ''y'' = 0 |- | Proof: || || ¬((''x'' ∨ ''y'') ∨ ''z'') ∧ ''y'' |- | || = || ¬((''y'' ∨ ''x'') ∨ ''z'') ∧ ''y'' || by '''Cmm<sub>1</sub>''' |- | || = || 0 || by '''H<sub>1</sub>''' |} | '''I<sub>2</sub>''' [dual] ¬((''x''∧''y'')∧''z'') ∨ ''y'' = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''J<sub>1</sub>''' !! !! ¬((''x''∨''y'')∨''z'') ∧ ''z'' = 0 |- | Proof: || || ¬((''x'' ∨ ''y'') ∨ ''z'') ∧ ''z'' |- | || = || (¬(''x'' ∨ ''y'') ∧ ¬''z'') ∧ ''z'' || by '''DMg<sub>1</sub>''' |- | || = || ''z'' ∧ (¬(''x'' ∨ ''y'') ∧ ¬''z'') || by '''Cmm<sub>2</sub>''' |- | || = || ''z'' ∧ ( ¬''z'' ∧ ¬(''x'' ∨ ''y'')) || by '''Cmm<sub>2</sub>''' |- | || = || 0 || by '''A<sub>2</sub>''' |} | '''J<sub>2</sub>''' [dual] ¬((''x''∧''y'')∧''z'') ∨ ''z'' = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''K<sub>1</sub>''' !! !! (''x'' ∨ (''y'' ∨ ''z'')) ∨ ¬((''x'' ∨ ''y'') ∨ ''z'') = 1 |- | Proof: || || (''x''∨(''y''∨''z'')) ∨ ¬((''x'' ∨ ''y'') ∨ ''z'') |- | || = || (''x''∨(''y''∨''z'')) ∨ (¬(''x'' ∨ ''y'') ∧ ¬''z'') || by '''DMg<sub>1</sub>''' |- | || = || (''x''∨(''y''∨''z'')) ∨ ((¬''x'' ∧ ¬''y'') ∧ ¬''z'') || by '''DMg<sub>1</sub>''' |- | || = || ((''x''∨(''y''∨''z'')) ∨ (¬''x'' ∧ ¬''y'')) ∧ ((''x''∨(''y''∨''z'')) ∨ ¬''z'')|| by '''Dst<sub>1</sub>''' |- | || = || (((''x''∨(''y''∨''z'')) ∨ ¬''x'') ∧ ((''x''∨(''y''∨''z'')) ∨ ¬''y'')) ∧ ((''x''∨(''y''∨''z'')) ∨ ¬''z'')|| by '''Dst<sub>1</sub>''' |- | || = || (1 ∧ 1) ∧ 1 || by '''D<sub>1</sub>''','''F<sub>1</sub>''','''G<sub>1</sub>''' |- | || = || 1 || by '''Idn<sub>2</sub>''' |} | '''K<sub>2</sub>''' [dual] (''x'' ∧ (''y'' ∧ ''z'')) ∧ ¬((''x'' ∧ ''y'') ∧ ''z'') = 0 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''L<sub>1</sub>''' !! !! (''x'' ∨ (''y'' ∨ ''z'')) ∧ ¬((''x'' ∨ ''y'') ∨ ''z'') = 0 |- | Proof: || || (''x'' ∨ (''y'' ∨ ''z'')) ∧ ¬((''x'' ∨ ''y'') ∨ ''z'') |- | || = || ¬((''x''∨''y'')∨''z'') ∧ (''x'' ∨ (''y'' ∨ ''z'')) || by '''Cmm<sub>2</sub>''' |- | || = || (¬((''x''∨''y'')∨''z'') ∧ ''x'') ∨ (¬((''x''∨''y'')∨''z'') ∧ (''y'' ∨ ''z'')) || by '''Dst<sub>2</sub>''' |- | || = || (¬((''x''∨''y'')∨''z'') ∧ ''x'') ∨ ((¬((''x''∨''y'')∨''z'') ∧ ''y'') ∨ (¬((''x''∨''y'')∨''z'') ∧ ''z'')) || by '''Dst<sub>2</sub>''' |- | || = || 0 ∨ (0 ∨ 0) || by '''H<sub>1</sub>''','''I<sub>1</sub>''','''J<sub>1</sub>''' |- | || = || 0 || by '''Idn<sub>1</sub>''' |} | '''L<sub>2</sub>''' [dual] (''x'' ∧ (''y'' ∧ ''z'')) ∨ ¬((''x'' ∧ ''y'') ∧ ''z'') = 1 |- valign="top" | {| align="left" class="collapsible collapsed" style="text-align:left" ! '''Ass<sub>1</sub>''' !! !! ''x'' ∨ (''y'' ∨ ''z'') = (''x'' ∨ ''y'') ∨ ''z'' |- | Proof: || || by '''K<sub>1</sub>''', '''L<sub>1</sub>''', '''UNg''', '''DNg''' |} | '''Ass<sub>2</sub>''' [dual] ''x'' ∧ (''y'' ∧ ''z'') = (''x'' ∧ ''y'') ∧ ''z'' |- | colspan="2" | {| align="left" class="collapsible" style="text-align:left" |- ! colspan="2" | Abbreviations |- | '''UId''' || Unique Identity |- | '''Idm''' || [[Idempotence]] |- | '''Bnd''' || [[Bounded lattice|Boundaries]] |- | '''Abs''' || [[Absorption law]] |- | '''UNg''' || Unique Negation |- | '''DNg''' || [[Double negation]] |- | '''DMg''' || [[De Morgan's Law]] |- | '''Ass''' || [[Associativity]] |} |} {| align="right" class="wikitable collapsible collapsed" style="text-align:left" ! colspan="4"| '''Huntington 1904 Boolean algebra axioms''' |- valign="top" | '''Idn<sub>1</sub>''' || ''x'' ∨ 0 = ''x'' | '''Idn<sub>2</sub>''' || ''x'' ∧ 1 = ''x'' |- valign="top" | '''Cmm<sub>1</sub>''' || ''x'' ∨ ''y'' = ''y'' ∨ ''x'' | '''Cmm<sub>2</sub>''' || ''x'' ∧ ''y'' = ''y'' ∧ ''x'' |- valign="top" | '''Dst<sub>1</sub>''' || ''x'' ∨ (''y''∧''z'') = (''x''∨''y'') ∧ (''x''∨''z'') | '''Dst<sub>2</sub>''' || ''x'' ∧ (''y''∨''z'') = (''x''∧''y'') ∨ (''x''∧''z'') |- valign="top" | '''Cpl<sub>1</sub>''' || ''x'' ∨ ¬''x'' = 1 | '''Cpl<sub>2</sub>''' || ''x'' ∧ ¬''x'' = 0 |- | colspan="4" | {| align="left" class="collapsible" style="text-align:left" |- ! colspan="2" | Abbreviations |- | '''Idn''' || [[Identity element|Identity]] |- | '''Cmm''' || [[Commutativity]] |- | '''Dst''' || [[Distributivity]] |- | '''Cpl''' || [[Complemented lattice|Complements]] |} |} The first axiomatization of Boolean lattices/algebras in general was given by the English philosopher and mathematician [[Alfred North Whitehead]] in 1898.{{sfn|Padmanabhan|Rudeanu|2008|p=[https://books.google.com/books?id=JlXSlpmlSv4C&pg=PA73 73]}}{{sfn|Whitehead|1898|p=37}} It included the [[#Definition|above axioms]] and additionally {{math|1=''x'' ∨ 1 = 1}} and {{math|1=''x'' ∧ 0 = 0}}. In 1904, the American mathematician [[Edward V. Huntington]] (1874–1952) gave probably the most parsimonious axiomatization based on {{math|1=∧}}, {{math|1=∨}}, {{math|1=¬}}, even proving the associativity laws (see box).{{sfn|Huntington|1904|pp=292-293}} He also proved that these axioms are [[Independence (mathematical logic)|independent]] of each other.{{sfn|Huntington|1904|p=296}} In 1933, Huntington set out the following elegant axiomatization for Boolean algebra.{{sfn|Huntington|1933a}} It requires just one binary operation {{math|1=+}} and a [[unary functional symbol]] {{math|1=''n''}}, to be read as 'complement', which satisfy the following laws: {{olist |1= ''Commutativity'': {{math|1=''x'' + ''y'' = ''y'' + ''x''}}. |2= ''Associativity'': {{math|1=(''x'' + ''y'') + ''z'' = ''x'' + (''y'' + ''z'')}}. |3= ''Huntington equation'': {{math|1=''n''(''n''(''x'') + ''y'') + ''n''(''n''(''x'') + ''n''(''y'')) = ''x''}}. }} [[Herbert Robbins]] immediately asked: If the Huntington equation is replaced with its dual, to wit: {{olist|start=4 |1= ''Robbins Equation'': {{math|1=''n''(''n''(''x'' + ''y'') + ''n''(''x'' + ''n''(''y''))) = ''x''}}, }} do (1), (2), and (4) form a basis for Boolean algebra? Calling (1), (2), and (4) a ''Robbins algebra'', the question then becomes: Is every Robbins algebra a Boolean algebra? This question (which came to be known as the [[Robbins conjecture]]) remained open for decades, and became a favorite question of [[Alfred Tarski]] and his students. In 1996, [[William McCune]] at [[Argonne National Laboratory]], building on earlier work by Larry Wos, Steve Winker, and Bob Veroff, answered Robbins's question in the affirmative: Every Robbins algebra is a Boolean algebra. Crucial to McCune's proof was the computer program [[Equational prover|EQP]] he designed. For a simplification of McCune's proof, see Dahn (1998). Further work has been done for reducing the number of axioms; see [[Minimal axioms for Boolean algebra]]. {{clear}}
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