Positive formula

A positive formula is a formula P such that $P\limp\oc P$ (thus a coalgebra for the comonad $\oc$). As a consequence P and $\oc P$ are equivalent.

A formula P is positive if and only if $P\orth$ is negative.

Positive connectives

A connective c of arity n is positive if for any positive formulas P₁,...,P_n, $c(P_1,\dots,P_n)$ is positive.

Proposition (Positive connectives)

$\tens$, $\one$, $\plus$, $\zero$, $\oc$ and $\exists$ are positive connectives.

Proof.  $\AxRule{P_2\vdash\oc{P_2}} \AxRule{P_1\vdash\oc{P_1}} \LabelRule{\rulename{ax}} \NulRule{P_1\vdash P_1} \LabelRule{\rulename{ax}} \NulRule{P_2\vdash P_2} \LabelRule{\tens R} \BinRule{P_1,P_2\vdash P_1\tens P_2} \LabelRule{\oc d L} \UnaRule{\oc{P_1},P_2\vdash P_1\tens P_2} \LabelRule{\oc d L} \UnaRule{\oc{P_1},\oc{P_2}\vdash P_1\tens P_2} \LabelRule{\oc R} \UnaRule{\oc{P_1},\oc{P_2}\vdash\oc{(P_1\tens P_2)}} \LabelRule{\rulename{cut}} \BinRule{P_1,\oc{P_2}\vdash\oc{(P_1\tens P_2)}} \LabelRule{\rulename{cut}} \BinRule{P_1,P_2\vdash\oc{(P_1\tens P_2)}} \LabelRule{\tens L} \UnaRule{P_1\tens P_2\vdash\oc{(P_1\tens P_2)}} \DisplayProof$

$\LabelRule{\one R} \NulRule{\vdash\one} \LabelRule{\oc R} \UnaRule{\vdash\oc{\one}} \LabelRule{\one L} \UnaRule{\one\vdash\oc{\one}} \DisplayProof$

$\AxRule{P_1\vdash\oc{P_1}} \LabelRule{\rulename{ax}} \NulRule{P_1\vdash P_1} \LabelRule{\plus_1 R} \UnaRule{P_1\vdash P_1\plus P_2} \LabelRule{\oc d L} \UnaRule{\oc{P_1}\vdash P_1\plus P_2} \LabelRule{\oc R} \UnaRule{\oc{P_1}\vdash\oc{(P_1\plus P_2)}} \LabelRule{\rulename{cut}} \BinRule{P_1\vdash\oc{(P_1\plus P_2)}} \AxRule{P_2\vdash\oc{P_2}} \LabelRule{\rulename{ax}} \NulRule{P_2\vdash P_2} \LabelRule{\plus_2 R} \UnaRule{P_2\vdash P_1\plus P_2} \LabelRule{\oc d L} \UnaRule{\oc{P_2}\vdash P_1\plus P_2} \LabelRule{\oc R} \UnaRule{\oc{P_2}\vdash\oc{(P_1\plus P_2)}} \LabelRule{\rulename{cut}} \BinRule{P_2\vdash\oc{(P_1\plus P_2)}} \LabelRule{\plus L} \BinRule{P_1\plus P_2\vdash\oc{(P_1\plus P_2)}} \DisplayProof$

$\LabelRule{\zero L} \NulRule{\zero\vdash\oc{\zero}} \DisplayProof$

$\LabelRule{\rulename{ax}} \NulRule{\oc{P}\vdash\oc{P}} \LabelRule{\oc R} \UnaRule{\oc{P}\vdash\oc{\oc{P}}} \DisplayProof$

$\AxRule{P\vdash\oc{P}} \LabelRule{\rulename{ax}} \NulRule{P\vdash P} \LabelRule{\exists R} \UnaRule{P\vdash \exists\xi P} \LabelRule{\oc d L} \UnaRule{\oc{P}\vdash \exists\xi P} \LabelRule{\oc R} \UnaRule{\oc{P}\vdash\oc{\exists\xi P}} \LabelRule{\rulename{cut}} \BinRule{P\vdash\oc{\exists\xi P}} \LabelRule{\exists L} \UnaRule{\exists\xi P\vdash\oc{\exists\xi P}} \DisplayProof$

More generally, $\oc A$ is positive for any formula A.

The notion of positive connective is related with but different from the notion of asynchronous connective.

Generalized structural rules

Positive formulas admit generalized left structural rules corresponding to a structure of $\tens$-comonoid: $P\limp P\tens P$ and $P\limp\one$. The following rule is derivable:

$\AxRule{\Gamma,P,P\vdash\Delta} \LabelRule{+ c L} \UnaRule{\Gamma,P\vdash\Delta} \DisplayProof \qquad \AxRule{\Gamma\vdash\Delta} \LabelRule{+ w L} \UnaRule{\Gamma,P\vdash\Delta} \DisplayProof$

Proof.  $\AxRule{P\vdash\oc{P}} \AxRule{\Gamma,P,P\vdash\Delta} \LabelRule{\oc L} \UnaRule{\Gamma,P,\oc P\vdash\Delta} \LabelRule{\oc L} \UnaRule{\Gamma,\oc P,\oc P\vdash\Delta} \LabelRule{\oc c L} \UnaRule{\Gamma,\oc P\vdash\Delta} \LabelRule{\rulename{cut}} \BinRule{\Gamma,P\vdash\Delta} \DisplayProof$

$\AxRule{P\vdash\oc{P}} \AxRule{\Gamma\vdash\Delta} \LabelRule{\oc w L} \UnaRule{\Gamma,\oc P\vdash\Delta} \LabelRule{\rulename{cut}} \BinRule{\Gamma,P\vdash\Delta} \DisplayProof$

Positive formulas are also acceptable in the left-hand side context of the promotion rule. The following rule is derivable:

$\AxRule{\oc\Gamma,P_1,\dots,P_n\vdash A,\wn\Delta} \LabelRule{+ \oc R} \UnaRule{\oc\Gamma,P_1,\dots,P_n\vdash \oc{A},\wn\Delta} \DisplayProof$

Proof.  $\AxRule{P_1\vdash\oc{P_1}} \AxRule{P_n\vdash\oc{P_n}} \AxRule{\oc\Gamma,P_1,\dots,P_n\vdash A,\wn\Delta} \LabelRule{\oc L} \UnaRule{\oc\Gamma,P_1,\dots,P_{n-1},\oc{P_n}\vdash A,\wn\Delta} \VdotsRule{}{\oc\Gamma,P_1,\oc{P_2},\dots,\oc{P_n}\vdash A,\wn\Delta} \LabelRule{\oc L} \UnaRule{\oc\Gamma,\oc{P_1},\dots,\oc{P_n}\vdash A,\wn\Delta} \LabelRule{\oc R} \UnaRule{\oc\Gamma,\oc{P_1},\dots,\oc{P_n}\vdash \oc{A},\wn\Delta} \LabelRule{\rulename{cut}} \BinRule{\oc\Gamma,\oc{P_1},\dots,\oc{P_{n-1}},P_n\vdash \oc{A},\wn\Delta} \VdotsRule{}{\oc\Gamma,\oc{P_1},P_2,\dots,P_n\vdash \oc{A},\wn\Delta} \LabelRule{\rulename{cut}} \BinRule{\oc\Gamma,P_1,\dots,P_n\vdash \oc{A},\wn\Delta} \DisplayProof$