The colimit algebra of the splitting chain #
A chain of objects with one-step transitions has a filtered colimit
over the v-small copy of ℕ. Given stagewise multiplications
compatible with the transitions, the colimit carries a multiplication;
given a bottom-stage unit with stagewise unit laws, it becomes a
monoid object, commutative when the stagewise multiplication is
commutative up to the index transport. The development is generic
over any monoidal category in which tensoring preserves the chain
colimits — the ind-category of a small monoidal category qualifies by
RS.tensorLeft_ind_preservesColimitsOfShape and its right-hand
twin — so the splitting chain of ChainDelta can be instantiated
later with B n := chainStage A M M' n and δ n := chainDelta.
The multiplication is assembled in two passes of colimit.desc
through the preservation isomorphisms, mirroring the merge pattern of
BigTensor: first against a fixed stage in the second slot, then over
the second slot. All colimit-level laws are cast-free because the
stage inclusions absorb the index transports.
Transport of a chain object along an equality of indices.
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The trivial index transport is the identity.
Index transports compose.
Index transports compose.
The chain diagram over the v-small copy of ℕ, the shape at
which the receiving category is assumed to have colimits.
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- RS.chainDiagram B δ = RS.smallNatEquiv.inverse.comp (RS.chainFunctor B δ)
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The colimit object of the chain.
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The stage inclusion into the chain colimit.
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The chain morphisms are absorbed by the stage inclusions.
The chain morphisms are absorbed by the stage inclusions.
The transitions are absorbed by the stage inclusions.
The transitions are absorbed by the stage inclusions.
The index transports are absorbed by the stage inclusions.
The index transports are absorbed by the stage inclusions.
Maps out of the chain colimit agree once they agree on all stages.
The colimit multiplication #
Stagewise multiplications compatible with the transitions assemble
into a multiplication on the chain colimit. The two compatibility
squares are taken as hypotheses; only the left one needs an index
transport, since (i + 1) + 1 + j is not definitionally
(i + 1 + j) + 1.
Multiplying after a chain morphism in the first slot agrees with multiplying first, once both land in the colimit.
Multiplying after a chain morphism in the second slot agrees with multiplying first, once both land in the colimit.
The multiply-then-include maps against a fixed stage in the second slot form a cocone on the chain diagram tensored on the right with that stage.
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Partial multiplication of the chain colimit against a fixed stage in the second slot.
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On a stage, the partial multiplication is multiply-then-include.
On a stage, the partial multiplication is multiply-then-include.
Maps out of the chain colimit tensored on the right are determined by their restrictions to the stages.
The partial multiplications are natural in the stage.
The partial multiplications are natural in the stage.
Maps out of the chain colimit tensored on the left are determined by their restrictions to the stages.
The partial multiplications form a cocone on the chain diagram tensored on the left with the chain colimit.
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The colimit multiplication: the partial multiplications assembled over the second slot.
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On a stage in the second slot, the colimit multiplication is the partial multiplication.
On a stage in the second slot, the colimit multiplication is the partial multiplication.
Defining equation of the colimit multiplication: on a pair of stages it is multiply-then-include.
Defining equation of the colimit multiplication: on a pair of stages it is multiply-then-include.
The multiply-then-include maps against a fixed stage in the first slot form a cocone on the chain diagram tensored on the left with that stage.
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Partial multiplication of a fixed stage in the first slot against the chain colimit.
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On a stage, the left partial multiplication is multiply-then-include.
On a stage, the left partial multiplication is multiply-then-include.
On a stage in the first slot, the colimit multiplication is the left partial multiplication.
On a stage in the first slot, the colimit multiplication is the left partial multiplication.
Maps out of the tensor square of the chain colimit are determined by pairs of stages.
Sandwich extensionality: maps out of a tensor product with the chain colimit in the middle slot are determined by the stages there.
The unit and the monoid laws #
The colimit unit is the bottom-stage unit followed by the stage inclusion. The monoid laws hold on the colimit whenever their stagewise forms hold; the index transports disappear into the stage inclusions.
The colimit unit: the bottom-stage unit followed by the stage inclusion.
Equations
- RS.chainColimitUnit B δ u = CategoryTheory.CategoryStruct.comp u (RS.chainColimitι B δ 0)
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Left unit law of the colimit multiplication, from the stagewise left unit law.
Right unit law of the colimit multiplication, from the stagewise right unit law.
Stagewise associativity, pushed into the colimit: the index transport is absorbed by the stage inclusion.
Associativity of the colimit multiplication, from stagewise associativity.
The chain colimit as a monoid object: the unit is the included bottom-stage unit and the multiplication is assembled from the stagewise multiplications.
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- RS.chainColimitMonObj B δ mu hδl hδr u hul hur hassoc = { one := RS.chainColimitUnit B δ u, mul := RS.chainColimitMul B δ mu hδl hδr, one_mul := ⋯, mul_one := ⋯, mul_assoc := ⋯ }
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Commutativity #
Stagewise commutativity, pushed into the colimit: the index transport is absorbed by the stage inclusion.
Commutativity of the colimit multiplication, from stagewise commutativity up to the index transport.
The chain colimit as a commutative monoid object: stagewise commutativity makes the packaged monoid structure commutative.
The ind-category instantiation #
Over the ind-category of a small monoidal category the generic
development applies verbatim: the shape has colimits, tensoring
preserves them (IndTensorExact), and the generic chain diagram is
the chain functor of ChainUnit, so the nonvanishing criterion for
the colimit unit transfers to the packaged unit.
Nonvanishing of the colimit unit: over the ind-category, the colimit unit built from a compatible family of stage units vanishes exactly when the family dies at a finite stage.