IBM Research | Ponder This | September 2002 solutions
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September 2002

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The answer, obtained by brute force (originally by hand), is that there are exactly 26 closed subsets, which result from appending the Identity to each of the following subsets of the other 11 operators:
the 16 subsets of T, F, And, Or
T, F, Not
T, Equivalent
F, Xor
T, F, Not, Equivalent, Xor
T, Or, Implies
F, And, Non-implies
T, Equivalent, Or, Implies, And
F, Xor, And, Non-implies, Or
all 11 operators

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