# Geometry of interaction

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The ''geometry of interaction'', GoI in short, was defined in the early nineties by Girard as an interpretation of linear logic into operators algebra: formulae were interpreted by Hilbert spaces and proofs by partial isometries. |
The ''geometry of interaction'', GoI in short, was defined in the early nineties by Girard as an interpretation of linear logic into operators algebra: formulae were interpreted by Hilbert spaces and proofs by partial isometries. |
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− | This was a striking novelty as it was the first time that a mathematical model of logic (lambda-calculus) didn't interpret a proof of <math>A\limp B</math> as a morphism ''from'' (the space interpreting) <math>A</math> ''to'' (the space interpreting) <math>B</math> and proof composition (cut rule) as the composition of morphisms. Rather the proof was interpreted as an operator acting ''on'' (the space interpreting) <math>A\limp B</math>, that is a morphism from <math>A\limp B</math> to <math>A\limp B</math>. For proof composition the problem was then, given an operator on <math>A\limp B</math> and another one on <math>B\limp C</math> to construct a new operator on <math>A\limp C</math>. This problem was originally expressed as a feedback equation solved by the ''execution formula''. The execution formula has some formal analogies with Kleene's formula for recursive functions, which allowed to claim that GoI was an ''operational semantics'', as opposed to traditionnal [[Semantics|denotational semantics]]. |
+ | This was a striking novelty as it was the first time that a mathematical model of logic (lambda-calculus) didn't interpret a proof of <math>A\limp B</math> as a morphism ''from'' <math>A</math> ''to'' <math>B</math> and proof composition (cut rule) as the composition of morphisms. Rather the proof was interpreted as an operator acting ''on'' <math>A\limp B</math>, that is a morphism from <math>A\limp B</math> to <math>A\limp B</math>. For proof composition the problem was then, given an operator on <math>A\limp B</math> and another one on <math>B\limp C</math> to construct a new operator on <math>A\limp C</math>. This problem was solved by the ''execution formula'' that bares some formal analogies with Kleene's formula for recursive functions. For this reason GoI was claimed to be an ''operational semantics'', as opposed to traditionnal [[Semantics|denotational semantics]]. |

The first instance of the GoI was restricted to the <math>MELL</math> fragment of linear logic (Multiplicative and Exponential fragment) which is enough to encode lambda-calculus. Since then Girard proposed several improvements: firstly the extension to the additive connectives known as ''Geometry of Interaction 3'' and more recently a complete reformulation using Von Neumann algebras that allows to deal with some aspects of [[Light linear logics|implicit complexity]] |
The first instance of the GoI was restricted to the <math>MELL</math> fragment of linear logic (Multiplicative and Exponential fragment) which is enough to encode lambda-calculus. Since then Girard proposed several improvements: firstly the extension to the additive connectives known as ''Geometry of Interaction 3'' and more recently a complete reformulation using Von Neumann algebras that allows to deal with some aspects of [[Light linear logics|implicit complexity]] |
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− | The GoI has been a source of inspiration for various authors. Danos and Regnier have reformulated the original model exhibiting its combinatorial nature using a theory of reduction of paths in proof-nets and showing the link with abstract machines; in particular the execution formula appears as the composition of two automata that interact one with the other through their common interface. Also the execution formula has rapidly been understood as expressing the composition of strategies in game semantics. It has been used in the theory of sharing reduction for lambda-calculus in the Abadi-Gonthier-Lévy reformulation and simplification of Lamping's representation of sharing. Finally the original GoI for the <math>MELL</math> fragment has been reformulated in the framework of traced monoidal categories following an idea originally proposed by Joyal. |
+ | The GoI has been a source of inspiration for various authors. Danos and Regnier have reformulated the original model exhibiting its combinatorial nature using a theory of reduction of paths in proof-nets and showing the link with abstract machines; the execution formula appears as the composition of two automata interacting through a common interface. Also the execution formula has rapidly been understood as expressing the composition of strategies in game semantics. It has been used in the theory of sharing reduction for lambda-calculus in the Abadi-Gonthier-Lévy reformulation and simplification of Lamping's representation of sharing. Finally the original GoI for the <math>MELL</math> fragment has been reformulated in the framework of traced monoidal categories following an idea originally proposed by Joyal. |

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− | = The Geometry of Interaction as an abstract machine = |
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= The Geometry of Interaction as operators = |
= The Geometry of Interaction as operators = |
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− | The original construction of GoI by Girard follows a general pattern already mentionned in [[coherent semantics]] under the name ''symmetric reducibility''. First set a general space in which the interpretations of proofs will live; here, in the case of GoI, the space is the space of bounded operators on <math>H=\ell^2</math>. |
+ | The original construction of GoI by Girard follows a general pattern already mentionned in the section on [[coherent semantics]] under the name ''symmetric reducibility'' and that was first put to use in [[phase semantics]]. First set a general space <math>P</math> called the ''proof space'' because this is where the interpretations of proofs will live. Make sure that <math>P</math> is a (not necessarily commutative) monoid. In the case of GoI, the proof space is a subset of the space of bounded operators on <math>\ell^2</math>. |

− | Second define a suitable duality on this space that will be denoted as <math>u\perp v</math>. For the GoI, two dualities have proved to work, the first one being nilpotency: two operators <math>u</math> and <math>v</math> are dual if <math>uv</math> is nilpotent, that is, if there is an nonegative integer <math>n</math> such that <math>(uv)^n = 0</math>. |
+ | Second define a particular subset of <math>P</math> that will be denoted by <math>\bot</math>; then derive a duality on <math>P</math>: for <math>u,v\in P</math>, <math>u</math> and <math>v</math> are dual<ref>In modern terms one says that <math>u</math> and <math>v</math> are ''polar''.</ref>iff <math>uv\in\bot</math>. |

− | Last define a ''type'' as a subset <math>T</math> of the proof space that is equal to its bidual: <math>T = T\biorth</math>. In the case of GoI this means that <math>u\in T</math> iff for all operator <math>v</math>, if <math>v\in T\orth</math>, that is if <math>u'v</math> is nilpotent for all <math>u'\in T</math>, then <math>u\perp v</math>, that is <math>uv</math> is nilpotent. |
+ | Such a duality defines an [[orthogonality relation]], with the usual derived definitions and properties. |

− | It remains now to interpret logical operations, that is associate a type to each formula, an object to each proof and show the adequacy lemma, if <math>u</math> is the interpretation of a proof of the formula <math>A</math> then <math>u</math> belongs to the type associated to <math>A</math>. |
+ | For the GoI, two dualities have proved to work; we will consider the first one: nilpotency, ''ie'', <math>\bot</math> is the set of nilpotent operators in <math>P</math>. Let us explicit this: two operators <math>u</math> and <math>v</math> are dual if there is a nonnegative integer <math>n</math> such that <math>(uv)^n = 0</math>. This duality is symmetric: if <math>uv</math> is nilpotent then <math>vu</math> is nilpotent also. |

− | == Preliminaries == |
+ | Last define a ''type'' as a subset <math>T</math> of the proof space that is equal to its bidual: <math>T = T\biorth</math>. This means that <math>u\in T</math> iff for all operator <math>v\in T\orth</math>, that is such that <math>u'v\in\bot</math> for all <math>u'\in T</math>, we have <math>uv\in\bot</math>. |

− | We begin by a brief tour of the operations on <math>H</math> that will be used in the sequel. |
+ | The real work<ref>The difficulty is to find the right duality that will make logical operations interpretable. General conditions that allows to achieve this have been formulated by Hyland and Schalk thanks to their theory of ''[[double glueing]]''.</ref>is now to interpret logical operations, that is to associate a type to each formula, an object to each proof and show the ''adequacy lemma'': if <math>u</math> is the interpretation of a proof of the formula <math>A</math> then <math>u</math> belongs to the type associated to <math>A</math>. |

− | Let us denote by <math>(e_n)_{n\in\mathbb{N}}</math> the canonical hilbertian basis of <math>H = \ell^2</math>; <math>e_n</math> is the sequence containing only 0's but at the <math>n</math>'s position where its value is <math>1</math>: <math>e_n = (\delta_{in})_{i\in\mathbb{N}}</math> where <math>\delta_{in}</math> is the standard Kroenecker function. Recall that the adjoint of an operator <math>u</math> is the operator <math>u^*</math> defined by <math>\langle u(x), y\rangle = \langle x, u^*(y)\rangle</math> for any <math>x,y\in H</math>. |
+ | == [[GoI for MELL: partial isometries|Partial isometries]] == |

− | A ''partial isometry'' is an operator <math>u</math> satisfying <math>uu^* u = u</math>; as a consequence <math>uu^*</math> is a projector the range of which is the range of <math>u</math>; we will call ''codomain'' the range of <math>u</math>. Similarly <math>u^* u</math> is also a projector the range of which is the ''domain'' of <math>u</math> (which is orthogonal to the kernel of <math>u</math>). The domain and the codomain of <math>u</math> are both closed subspace of <math>H</math> and <math>u</math> restricted to its domain is an isometry. If the domain <math>u</math> is <math>H</math> that is if <math>u^* u = 1</math> we say that <math>u</math> has ''full domain'', and similarly for codomain. |
+ | The first step is to build the proof space. This is constructed as a special set of partial isometries on a separable Hilbert space <math>H</math> which turns out to be generated by partial permutations on the canonical basis of <math>H</math>. |

− | If <math>u</math> is a partial isometry then <math>u^*</math> is also a partial isometry the domain of which is the codomain of <math>u</math> and the codomain of which is the domain of <math>u</math>. |
+ | These so-called ''<math>p</math>-isometries'' enjoy some nice properties, the most important one being that a <math>p</math>-isometry is a sum of <math>p</math>-isometries iff all the terms of the sum have disjoint domains and disjoint codomains. As a consequence we get that a sum of <math>p</math>-isometries is null iff each term of the sum is null. |

− | If <math>u</math> and <math>v</math> are two partial isometries, the equation <math>uu^* + vv^* = 1</math> means that the codomains of <math>u</math> and <math>v</math> are orthogonal and that their direct sum is <math>H</math>. |
+ | A second important property is that operators on <math>H</math> can be ''externalized'' using <math>p</math>-isometries into operators acting on <math>H\oplus H</math>, and conversely operators on <math>H\oplus H</math> may be ''internalized'' into operators on <math>H</math>. This is widely used in the sequel. |

− | We shall define a number of operators on <math>H</math> by describing their action on the basis <math>(e_n)</math>; actually most of the operators that are used to interpret logical operations will turn out to be defined as partial permutations on the basis, which in particular entails that they are partial isometries. More precisely given a partial permutation <math>\varphi</math> on <math>\mathbb{N}</math>, that is a one-to-one function from a subset of <math>\mathbb{N}</math> into <math>\mathbb{N}</math> one define the partial isometry <math>u_\varphi</math> by <math>u_\varphi(e_n) = e_{\varphi(n)}</math> if <math>\varphi(n)</math> is defined, <math>0</math> otherwise. |
+ | == [[GoI for MELL: the *-autonomous structure|The *-autonomous structure]] == |

− | == Interpreting the tensor == |
+ | The second step is to interpret the linear logic multiplicative operations, most importantly the cut rule. |

− | The first step is, given two types <math>A</math> and <math>B</math>, to construct the type <math>A\tens B</math>. For this purpose we will define an isomorphism <math>H\oplus H \cong H</math> by <math>x\oplus y\rightsquigarrow p(x)+q(y)</math> where <math>p:H\mapsto H</math> and <math>q:H\mapsto H</math> are partial isometries given by: |
+ | Internalization/externalization is the key for this: typically the type <math>A\tens B</math> is interpreted by a set of <math>p</math>-isometries which are internalizations of operators acting on <math>H\oplus H</math>. |

− | : <math>p(e_n) = e_{2n}</math>, |
+ | The (interpretation of) the cut-rule is defined in two steps: firstly we use nilpotency to define an operation corresponding to lambda-calculus application which given two <math>p</math>-isometries in respectively <math>A\limp B</math> and <math>A</math> produces an operator in <math>B</math>. From this we deduce the composition and finally obtain a structure of *-autonomous category, that is a model of multiplicative linear logic. |

− | : <math>q(e_n) = e_{2n+1}</math>. |
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− | This is actually arbitrary, any two partial isometries <math>p,q</math> with full domain and such that the sum of their codomains is <math>H</math> would do the job. |
+ | == [[GoI for MELL: exponentials|The exponentials]] == |

− | From the definition <math>p</math> and <math>q</math> have full domain, that is satisfy <math>p^* p = q^* q = 1</math>. On the other hand their codomains are orthogonal, thus we have <math>p^* q = q^* p = 0</math>. Note that we also have <math>pp^* + qq^* = 1</math> although this property is not needed in the sequel. |
+ | Finally we turn to define exponentials, that is connectives managing duplication. To do this we introduce an isomorphism (induced by a <math>p</math>-isometry) between <math>H</math> and <math>H\tens H</math>: the first component of the tensor is intended to hold the address of the the copy whereas the second component contains the content of the copy. |

− | Let <math>U</math> be an operator on <math>H\oplus H</math>. We can write <math>U</math> as a matrix: |
+ | We eventually get a quasi-model of full MELL; quasi in the sense that if we can construct <math>p</math>-isometries for usual structural operations in MELL (contraction, dereliction, digging), the interpretation of linear logic proofs is not invariant w.r.t. cut elimination in general. It is however invariant in some good cases, which are enough to get a correction theorem for the interpretation. |

− | : <math>U = \begin{pmatrix} |
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− | U_{11} & U_{12}\\ |
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− | U_{21} & U_{22} |
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− | \end{pmatrix}</math> |
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− | where each <math>U_{ij}</math> operates on <math>H</math>. |
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− | Now through the isomorphism <math>H\oplus H\cong H</math> we may transform <math>U</math> into the operator <math>\bar U</math> on <math>H</math> defined by: |
+ | = The Geometry of Interaction as an abstract machine = |

− | |||

− | : <math>\bar U = pU_{11}p^* + pU_{12}q^* + qU_{21}p^* + qu_{22}q^*</math>. |
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− | |||

− | We call <math>\bar U</math> the ''internalization'' of <math>U</math>. |
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− | |||

− | Given <math>A</math> and <math>B</math> two types, we define their tensor by: |
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− | : <math>A\tens B = \{pup^* + qvq^*, u\in A, v\in B\}\biorth</math> |
+ | = Notes and references = |

− | From what precedes we see that <math>A\tens B</math> is generated by the internalizations of operators on <math>H\oplus H</math> of the form: |
+ | <references/> |

− | : <math>\begin{pmatrix} |
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− | u & 0\\ |
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− | 0 & v |
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− | \end{pmatrix}</math> |

## Latest revision as of 15:39, 30 September 2011

The *geometry of interaction*, GoI in short, was defined in the early nineties by Girard as an interpretation of linear logic into operators algebra: formulae were interpreted by Hilbert spaces and proofs by partial isometries.

This was a striking novelty as it was the first time that a mathematical model of logic (lambda-calculus) didn't interpret a proof of as a morphism *from* *A* *to* *B* and proof composition (cut rule) as the composition of morphisms. Rather the proof was interpreted as an operator acting *on* , that is a morphism from to . For proof composition the problem was then, given an operator on and another one on to construct a new operator on . This problem was solved by the *execution formula* that bares some formal analogies with Kleene's formula for recursive functions. For this reason GoI was claimed to be an *operational semantics*, as opposed to traditionnal denotational semantics.

The first instance of the GoI was restricted to the *M**E**L**L* fragment of linear logic (Multiplicative and Exponential fragment) which is enough to encode lambda-calculus. Since then Girard proposed several improvements: firstly the extension to the additive connectives known as *Geometry of Interaction 3* and more recently a complete reformulation using Von Neumann algebras that allows to deal with some aspects of implicit complexity

The GoI has been a source of inspiration for various authors. Danos and Regnier have reformulated the original model exhibiting its combinatorial nature using a theory of reduction of paths in proof-nets and showing the link with abstract machines; the execution formula appears as the composition of two automata interacting through a common interface. Also the execution formula has rapidly been understood as expressing the composition of strategies in game semantics. It has been used in the theory of sharing reduction for lambda-calculus in the Abadi-Gonthier-Lévy reformulation and simplification of Lamping's representation of sharing. Finally the original GoI for the *M**E**L**L* fragment has been reformulated in the framework of traced monoidal categories following an idea originally proposed by Joyal.

## Contents |

# [edit] The Geometry of Interaction as operators

The original construction of GoI by Girard follows a general pattern already mentionned in the section on coherent semantics under the name *symmetric reducibility* and that was first put to use in phase semantics. First set a general space *P* called the *proof space* because this is where the interpretations of proofs will live. Make sure that *P* is a (not necessarily commutative) monoid. In the case of GoI, the proof space is a subset of the space of bounded operators on .

Second define a particular subset of *P* that will be denoted by ; then derive a duality on *P*: for , *u* and *v* are dual^{[1]}iff .

Such a duality defines an orthogonality relation, with the usual derived definitions and properties.

For the GoI, two dualities have proved to work; we will consider the first one: nilpotency, *ie*, is the set of nilpotent operators in *P*. Let us explicit this: two operators *u* and *v* are dual if there is a nonnegative integer *n* such that (*u**v*)^{n} = 0. This duality is symmetric: if *u**v* is nilpotent then *v**u* is nilpotent also.

Last define a *type* as a subset *T* of the proof space that is equal to its bidual: . This means that iff for all operator , that is such that for all , we have .

The real work^{[2]}is now to interpret logical operations, that is to associate a type to each formula, an object to each proof and show the *adequacy lemma*: if *u* is the interpretation of a proof of the formula *A* then *u* belongs to the type associated to *A*.

## [edit] Partial isometries

The first step is to build the proof space. This is constructed as a special set of partial isometries on a separable Hilbert space *H* which turns out to be generated by partial permutations on the canonical basis of *H*.

These so-called * p-isometries* enjoy some nice properties, the most important one being that a

*p*-isometry is a sum of

*p*-isometries iff all the terms of the sum have disjoint domains and disjoint codomains. As a consequence we get that a sum of

*p*-isometries is null iff each term of the sum is null.

A second important property is that operators on *H* can be *externalized* using *p*-isometries into operators acting on , and conversely operators on may be *internalized* into operators on *H*. This is widely used in the sequel.

## [edit] The *-autonomous structure

The second step is to interpret the linear logic multiplicative operations, most importantly the cut rule.

Internalization/externalization is the key for this: typically the type is interpreted by a set of *p*-isometries which are internalizations of operators acting on .

The (interpretation of) the cut-rule is defined in two steps: firstly we use nilpotency to define an operation corresponding to lambda-calculus application which given two *p*-isometries in respectively and *A* produces an operator in *B*. From this we deduce the composition and finally obtain a structure of *-autonomous category, that is a model of multiplicative linear logic.

## [edit] The exponentials

Finally we turn to define exponentials, that is connectives managing duplication. To do this we introduce an isomorphism (induced by a *p*-isometry) between *H* and : the first component of the tensor is intended to hold the address of the the copy whereas the second component contains the content of the copy.

We eventually get a quasi-model of full MELL; quasi in the sense that if we can construct *p*-isometries for usual structural operations in MELL (contraction, dereliction, digging), the interpretation of linear logic proofs is not invariant w.r.t. cut elimination in general. It is however invariant in some good cases, which are enough to get a correction theorem for the interpretation.

# [edit] The Geometry of Interaction as an abstract machine

# [edit] Notes and references

- ↑ In modern terms one says that
*u*and*v*are*polar*. - ↑ The difficulty is to find the right duality that will make logical operations interpretable. General conditions that allows to achieve this have been formulated by Hyland and Schalk thanks to their theory of
*double glueing*.