Let $L=(X,\wedge,\vee,^\bot)$ denote a non-empty lattice having a unary operation $x \mapsto x^\bot$ that satisfies both "shuffle" laws: $$x \wedge y \leq z \iff x \leq y^\bot \vee z.$$
$$x \leq y \vee z \iff x \wedge y^\bot \leq z.$$
Then $L$ is an orthocomplemented lattice. (Proof below).
Question. Does every orthocomplemented lattice satisfy the shuffle laws? It suffices to prove just one of them, but I've been unable to do so.
Proof of claim.
Complements. Let $x$ be fixed but arbitrary. This is allowed, since $L$ was assumed non-empty. We will show that $x^\bot \wedge x$ is a least element. TFAE.
- $x^\bot \wedge x \leq a$
- $x^\bot \leq x^\bot \vee a$
- TRUE
Thus $x^\bot \wedge x$ is a least element. A similar argument shows that $x^\bot \vee x$ is a maximum element.
Involutiveness. We will show that $x \leq x^{\bot\bot}.$ TFAE.
- $x \leq a$
- $x \wedge x \leq a$
- $x \leq x^\bot \vee a$
- $x \wedge x^{\bot\bot} \leq a$
So the elements above $x$ and $x \wedge x^{\bot\bot}$ are identical; by antisymmetry, $x = x \wedge x^{\bot\bot}$. It follows that $x \leq x^{\bot\bot}.$
A similar argument shows the other direction; therefore $x = x^{\bot\bot}.$
Order-reversingness. The following are equivalent.
- $x \leq y$
- $x \wedge 1 \leq y$
- $1 \leq x^\bot \vee y$
- $1 \wedge y^\bot \leq x^\bot$
- $y^\bot \leq x^\bot$