nLab minor scale

Contents

Contents

Idea

The idea of a minor scale comes from Peter Freyd, in his attempt to give an algebraic (in the sense of universal algebra) description of the real interval, which he believed to be more fundamental than the real numbers themselves in analysis.

Certain minor scales are models of multiplicative linear logic.

Minor scales are an example of centipede mathematics; a lot of this page will be determining which of Freyd’s results in Algebraic Real Analysis still applies in minor scales and possible weakenings of the scale identity axiom.

Note: The terms multiplicative conjunction, multiplicative disjunction, linear implication, reverse linear implication, additive conjunction, and additive disjunction used in this page come from scales, which are algebraic models of multiplicative-additive linear logics. While the same operations may be defined in the same way in minor scales as in scales, the operations may no longer correspond to any linear logic at all, and are simply defined to reduce syntactical complexity in equational sentences and axioms.

Definition

In terms of multiplicative conjunction and disjunction

A minor scale is a symmetric closed midpoint algebra (M,|,,() ,,)(M,\vert, \odot, (-)^\bullet, \bot, \top) with two binary operations ()():M×MM(-)\otimes(-):M\times M\to M called multiplicative conjunction and ()():M×MM(-)\oplus(-):M\times M\to M called multiplicative disjunction such that for all aa in MM,

ab=baa \otimes b = b \otimes a
a=\bot \otimes a = \bot
a=aa \otimes \top = a
ab=baa \oplus b = b \oplus a
a=a\bot \oplus a = a
a=a \oplus \top = \top

In terms of linear implication

Note: linear implication is usually denoted using \multimap. We use \rightarrowtail instead because there is no left-pointing \multimap in LaTeX? for the dual linear implication, while there is \leftarrowtail

From \rightarrowtail one could define the zoom operators.

\bot-zooming can be defined as

a a aa^\vee \coloneqq a^\bullet \rightarrowtail a

\top-zooming can be defined as

a (aa ) a^\wedge \coloneqq (a \rightarrowtail a^\bullet)^\bullet

In terms of zoom operations

A minor scale is a symmetric closed midpoint algebra (M,|,,() ,,)(M,\vert, \odot, (-)^\bullet, \bot, \top) with two functions () :MM(-)^\wedge:M\to M called \top-zooming and () :MM(-)^\vee:M\to M called \bot-zooming such that for all aa in MM,

(|a) =(\bot \vert a)^\wedge = \bot
(|a) =a(\bot \vert a)^\vee = a
(a|) =a(a \vert \top)^\wedge = a
(a|) =(a \vert \top)^\vee = \top

In terms of central dilatations

A minor scale is a symmetric closed midpoint algebra (M,|,,() ,,)(M,\vert, \odot, (-)^\bullet, \bot, \top) with a function () :MM(-)^\dagger:M\to M called central dilatation such that for all aa in MM,

(|a) =a(\odot \vert a)^\dagger = a
(|(|a)) =(\top \vert (\top \vert a))^\dagger = \top
(|(|a)) =(\bot \vert (\bot \vert a))^\dagger = \bot

\bot-zooming can be defined as

a ((a|(|)) ) a^\vee \coloneqq ((a \vert (\odot \vert \top))^\dagger)^\dagger

\top-zooming can be defined as

a (((|)|a) ) a^\wedge \coloneqq (((\bot \vert \odot) \vert a)^\dagger)^\dagger

Multiplicative conjunction can be defined as

ab(a|b) a \otimes b \coloneqq (a \vert b)^\wedge

Multiplicative disjunction can be defined as

ab(a|b) a \oplus b \coloneqq (a \vert b)^\vee

Linear implication can be defined as

aba ba \multimap b \coloneqq a^\bullet \oplus b

Other operations

The binary operation additive conjunction is defined as

aba(a b)a \wedge b \coloneqq a \otimes (a^\bullet \oplus b)

and additive disjunction is defined as

aba(a b)a \vee b \coloneqq a \oplus (a^\bullet \otimes b)

The binary operation dilatation is defined as

ab((a |)b) a\vartriangleleft b \coloneqq ((a^\bullet \vert \bot)\oplus b)^\wedge

or as

ab((a |)b) a\vartriangleleft b \coloneqq ((a^\bullet \vert \top)\otimes b)^\vee

Defining operations in terms of other operations

Multiplication conjunction, multiplicative disjunction, \top-zooming, \bot-zooming, linear implication, and dual linear implication can all be defined in terms of each other, |\vert, and () (-)^\bullet.

operation a b a \otimes b a b a \oplus b a a^\wedge a a^\vee a b a \leftarrowtail b a b a \rightarrowtail b
a b a \otimes b a b a \otimes b ( a b ) (a^\bullet \oplus b^\bullet)^\bullet ( a | b ) (a\vert b)^\wedge ( ( a | b ) ) ((a^\bullet \vert b^\bullet)^\vee)^\bullet a b a^\bullet \leftarrowtail b ( a b ) (a \rightarrowtail b^\bullet)^\bullet
a b a \oplus b ( a b ) (a^\bullet \otimes b^\bullet)^\bullet a b a \oplus b ( ( a | b ) ) ((a^\bullet \vert b^\bullet)^\wedge)^\bullet ( a | b ) (a \vert b)^\vee ( a b ) (a \leftarrowtail b^\bullet)^\bullet a b a^\bullet \rightarrowtail b
a a^\wedge a a a \otimes a ( a a ) (a^\bullet \oplus a^\bullet)^\bullet a a^\wedge ( ( a ) ) ((a^\bullet)^\vee)^\bullet a a a^\bullet \leftarrowtail a ( a a ) (a \rightarrowtail a^\bullet)^\bullet
a a^\vee ( a a ) (a^\bullet \otimes a^\bullet)^\bullet a a a \oplus a ( ( a ) ) ((a^\bullet)^\wedge)^\bullet a a^\vee ( a a ) (a \leftarrowtail a^\bullet)^\bullet a a a^\bullet \rightarrowtail a
a b a \leftarrowtail b a b a^\bullet \otimes b ( a b ) (a \oplus b^\bullet)^\bullet ( a | b ) (a^\bullet \vert b)^\wedge ( ( a | b ) ) ((a \vert b^\bullet)^\vee)^\bullet a b a \leftarrowtail b ( a b ) (a^\bullet \rightarrowtail b^\bullet)^\bullet
a b a \rightarrowtail b ( a b ) (a \otimes b^\bullet)^\bullet a b a^\bullet \oplus b ( ( a | b ) ) ((a \vert b^\bullet)^\wedge)^\bullet ( a | b ) (a^\bullet \vert b)^\vee ( a b ) (a^\bullet \leftarrowtail b^\bullet)^\bullet a b a \rightarrowtail b

Additional possible equational axioms

Here is a list of other possible equational axioms that a minor scale might satisfy:

  • the scale identities: for all aa and bb in MM,
ab=(a b )|(a b )a \otimes b = (a^\vee \otimes b^\wedge) \vert (a^\wedge \otimes b^\vee)
ab=(a b )|(a b )a \oplus b = (a^\wedge \oplus b^\vee) \vert (a^\vee \oplus b^\wedge)

The rest of the axioms follow from the scale identities:

=\top^\vee = \top
  • \bot is a fixed point of () (-)^\wedge:
=\bot^\wedge = \bot
  • the axiom of compensation: for all aa in MM,
a=a |a a = a^\vee \vert a^\wedge
  • the axioms of central distributivity: for all aa in MM,
a=a |a \otimes \odot = a^\wedge \vert \bot
a=a |a \oplus \odot = a^\vee \vert \top
  • the axiom of balance: for all aa and bb in MM,
a|(ab)=b|(ba)a \vert (a \multimap b) = b \vert (b \multimap a)
  • the disjunction property: for all aa and bb in MM,
(ab)(ba)=(a \rightarrowtail b) \vee (b \rightarrowtail a) = \top
  • the coalgebra equation: for all aa in MM,
a (a ) =a^\vee \vee (a^\wedge)^\bullet = \top
  • associativity of multiplicative conjunction: for all aa, bb, and cc in MM
a(bc)=(ab)ca \otimes (b \otimes c) = (a \otimes b) \otimes c
  • associativity of multiplicative disjunction: for all aa, bb, and cc in MM
a(bc)=(ab)ca \oplus (b \oplus c) = (a \oplus b) \oplus c
  • the various lattice axioms for (M,,,,)(M,\top,\bot,\wedge,\vee): for all aa, bb, and cc in MM,
aa=aa \wedge a = a
ab=baa \wedge b = b \wedge a
a(bc)=(ab)ca \wedge (b \wedge c) = (a \wedge b) \wedge c
a=a\top \wedge a = a
aa=aa \vee a = a
ab=baa \vee b = b \vee a
a(bc)=(ab)ca \vee (b \vee c) = (a \vee b) \vee c
a=a\bot \vee a = a
a(ab)=aa \wedge (a \vee b) = a
a(ab)=aa \vee (a \wedge b) = a
  • for all aa and bb in MM,
(ab)(a |b )=(a \otimes b) \rightarrowtail (a^\wedge \vert b^\wedge) = \top
  • for all aa and bb in MM,
(ab)(a |b )= (a \oplus b) \leftarrowtail (a^\vee \vert b^\vee) = \bot

Properties

Every minor scale with =\bot = \top is trivial.

\bot is a fixed point of () (-)^\vee and \top is a fixed point of () (-)^\wedge.

As a minor scale is a closed midpoint algebra, a minor scale has a partial order \leq. If aba \leq b, then ab=a \multimap b = \top.

The currying of \vartriangleleft results in a dilatation at an element ():M(MM)(-)\vartriangleleft:M\to (M\to M). For every element aa in MM, dilatation at aa is a retraction of contraction at aa: (a)(a|)=id M(a\vartriangleleft) \circ (a\vert) = id_M, where id Mid_M is the identity function on MM.

Examples

The unit interval with a|ba+b2a \vert b \coloneqq \frac{a + b}{2}, =12\odot = \frac{1}{2}, a =1aa^\bullet = 1 - a, =0\bot = 0, =1\top = 1, ab=max(a+b1,0)a \otimes b = max(a + b - 1,0), ab=min(a+b,1)a \oplus b = min(a + b,1), a =max(2a1,0)a^\wedge = max(2a - 1,0) and a =min(2a,1)a^\vee = min(2a,1) is an example of a minor scale.

The set of truth values in Girard’s linear logic is a minor scale.

References

  • Peter Freyd, Algebraic real analysis, Theory and Applications of Categories, Vol. 20, 2008, No. 10, pp 215-306 (tac:20-10)

Last revised on June 3, 2021 at 16:01:05. See the history of this page for a list of all contributions to it.