Math 2.0

Order, lattices, ordered algebraic structures Definition

Huntington's axioms

Builds on Boolean algebra

Definition

Let AA be a set with a binary operation ++ and a unary operation nn. The structure (A,+,n)(A, +, n) satisfies Huntington's axioms if for all x,y,z∈Ax, y, z \in A:

  1. x+y=y+xx + y = y + x.
  2. (x+y)+z=x+(y+z)(x + y) + z = x + (y + z).
  3. n(n(x)+y)+n(n(x)+n(y))=xn(n(x) + y) + n(n(x) + n(y)) = x.

The third axiom is called Huntington's equation.

Remarks

A structure satisfies Huntington's axioms if and only if it comes from a Boolean algebra with x+y=x∨yx + y = x \vee y and n(x)=¬xn(x) = \neg x. The other operations are recovered as x∧y=n(n(x)+n(y))x \wedge y = n(n(x) + n(y)), 1=x+n(x)1 = x + n(x) and 0=n(1)0 = n(1), where the last two do not depend on xx.

Example: {0,1}\set{0, 1} with x+y=x∨yx + y = x \vee y and n(x)=¬xn(x) = \neg x. By De Morgan's laws, the left side of axiom 3 is (x∧¬y)∨(x∧y)=x∧(¬y∨y)=x(x \wedge \neg y) \vee (x \wedge y) = x \wedge (\neg y \vee y) = x.

Non-example: {0,1}\set{0, 1} with x+y=x∨yx + y = x \vee y and n(x)=xn(x) = x. Axioms 1 and 2 hold, but axiom 3 becomes (x∨y)∨(x∨y)=x(x \vee y) \vee (x \vee y) = x, which fails for x=0x = 0, y=1y = 1.

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Further reading

  1. Web https://en.wikipedia.org/wiki/Boolean_algebra_(structure)
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