1Introduction ContinuousSemiring-SemimodulePairsandMixedAlgebraicSystems

Continuous Semiring-Semimodule Pairs and Mixed Algebraic Systems ^∗

Zolt´ an ´ Esik

and Werner Kuich

Abstract

We associate with every commutative continuous semiringSand alphabet Σ a category whose objects are all sets and a morphismX→Y is determined by a function fromX into the semiring of formal seriesS⟪(Y⊎Σ)^∗⟫of finite words overY ⊎Σ, anX×Y-matrix overS⟪(Y⊎Σ)^∗⟫, and a function from X into the continuousS⟪(Y ⊎Σ)^∗⟫-semimoduleS⟪(Y⊎Σ)^ω⟫of series ofω- words overY⊎Σ. WhenS is also anω-semiring (equipped with an infinite product operation), then we define a fixed point operation over our category and show that it satisfies all identities of iteration categories. We then use this fixed point operation to give semantics to recursion schemes defining series of finite and infinite words. In the particular case when the semiring is the Boolean semiring, we obtain the context-free languages of finite andω-words.

1 Introduction

Suppose that S is a continuous semiring and Σ and X are sets. Let X^∗ and X^ω respectively denote the sets of all finite and ω-words over X. We can form the continuous semiring S⟪X^∗⟫ of series over X^∗ with coefficients in S and the continuous S⟪X^∗⟫-semimodule S⟪X^ω⟫ of series over X^ω with coefficients in S.

If S is equipped with an infinite product operation S^ω → S, s1s2⋯ ↦ ∏ⁿ≥1sn, satisfying certain axioms including a sort of continuity described in the sequel, then we can also define an infinite product operation(S⟪X^∗⟫)^ω→S⟪X^ω⟫. In our first result, we show that the construction of the ‘continuous ω-semiring-semimodule pair’(S⟪X^∗⟫,S⟪X^ω⟫)enjoys a universal property, cf. Theorem 5.1.

In the second part of the paper we use the above universality result to give an algebraic treatment of recursion schemes defining series of finite and infinite words over Σ. To this end, we will restrict ourselves to commutative continuous

∗The first author received support from NKFI grant no. ANN 110883. The second author was partially supported by Austrian Science Fund (FWF): grant no. I1661 -N25.

aDept. of Foundations of Computer Science, University of Szeged, Szeged, Hungary

bInst. of Discrete Mathematics and Geometry, Technical University of Vienna, Vienna, Austria E-mail:kuich@tuwien.ac.at

DOI: 10.14232/actacyb.23.1.2017.5

ω-semiringsS. Suppose that

xi = pii∈I (R)

is a finite or infinite recursion scheme (or system of fixed point equations), where eachpi is a series inS⟪(X⊎Σ)^∗⟫. Then we associate withR in a natural way a function

FR∶ S⟪Σ^∗⟫ ×S⟪S^ω⟫ →S⟪Σ^∗⟫ ×S⟪S^ω⟫ and define the semantics ofRas a ‘canonical’ fixed point ofFR.

In order to facilitate the construction of the canonical fixed point, we introduce a category whose objects are sets (of recursion variables) and a morphismf : X→Y has three components:

• a functionf0∶X→S⟪(Y ⊎Σ)^∗⟫,

• a matrixfM ∈ (S⟪(Y ⊎Σ)^∗⟫)^X×Y,

• a functionf_ω : X→S⟪(Y ⊎Σ)^ω⟫.

We define composition and the identity morphisms in a natural way to obtain a category Ser^ω_S,Σ. By taking the first components of morphisms, this category can be projected onto the category SerS,Σ having sets as objects and functions X →S⟪(Y ⊎Σ)^∗⟫ as morphismsX →Y. It will be clear from the definition that the coproductX1⊕ ⋯ ⊕Xn of any finite sequence X1, . . . , Xn of objects exists in Ser^ω_S,Σ, in fact it will be given by disjoint union. (Actually all coproducts will exist, but this fact is not important for the paper.)

Next we define a dagger operation mapping a morphism f : X → X⊕Y to a morphism f^† : X → Y. We prove that for any f : X → X⊕Y, the morphism f^†∶X →Y is a solution of the fixed point equation

ξ = f○ ⟨ξ,idY⟩.

(Here, ⟨−,−⟩ denotes the source pairing operation determined by the coproduct structure.) Intuitively, f represents a system of fixed point equations (recursion scheme) in the variables X and parameters Y, and f^† is its canonical solution.

In particular, the function F_R associated with a system of fixed point equations (R) can be seen as a morphism f : X → X in Ser^ω_S,Σ, and the canonical fixed point ofFR can be derived fromf^† : X → ∅. Indeed the three components off^† are a function X →S⟪Σ^∗⟫, the empty X× ∅-matrix overS⟪Σ^∗⟫, and a function X→S⟪Σ^ω⟫. The first and third components form the canonical fixed point ofF_R. Our approach generalizes the construction of context-free languages of finite and ω-words [10].

Categories with finite coproducts (or dually, products) equipped with a para- metric fixed point operation have been studied since the late 1960’s. A class of structures, called iteration theories, or iteration categories, have been identified. It has been shown that most (if not all) of the major fixed point structures used in computer science give rise to iteration categories. As a main technical contribution,

we prove that for each commutative continuous semiringSand alphabet Σ,Ser^ω_S,Σ is also an iteration category, cf. Theorem 7.1. A few consequences of this fact are discussed in the conclusion.

2 Continuous semirings

Recall that a commutative monoid(V,+,0)iscontinuous(cf. Section 2.2 of [6]) if it is equipped with a a partial order ≤ such that the supremum of any chain (or equivalently, directed set [12]) exists and 0 is the least element. Moreover, the sum operation+is continuous:

x+supY = sup(x+Y)

for all nonempty chains or nonempty directed setsY ⊆V, where x+Y = {x+y ∶ y∈ Y}. (Actually this also holds when the set is empty.) It follows that the sum operation is monotonic: ifx≤y in V, thenx+z≤y+z for allz∈V.

Suppose that V is a continuous commutative monoid andx_i ∈V for all i∈I.

We define∑i∈Ix_i as the supremum of all finite sumsx_i1+. . .+x_in wherei₁, . . . , i_n are pairwise different elements ofI. It is well-known that this summation operation is completely associative and commutative:

∑

i∈I

x_i = ∑

j∈J

∑

i∈I_j

x_i,

whenever I is the disjoint union of the sets I_j, j ∈ J and x_i ∈ V for all i ∈ I.

Moreover,

∑

i∈I

xi = ∑

i∈I

x_π(i),

wheneverπis a permutation I→I andxi∈V for alli∈I.

Continuous commutative monoids are closed under several constructions includ- ing direct product. Suppose that V_i is a continuous commutative monoid for all i∈I. ThenV = ∏ⁱ∈IV_i, equipped with the pointwise sum operation and pointwise ordering, is also a continuous commutative monoid. It follows that the summation operation in the product V is the pointwise summation. In particular, if V is a continuous commutative monoid, then so isV^I for any set I.

Suppose now that S = (S,+,⋅,0,1) is a semiring [8, 11]. We say that S is a continuous semiring(cf. Section 2.2 of [6]) if(S,+,0)is a continuous commutative monoid equipped with a partial order≤¹ and the product operation is continuous (hence, also monotonic), i.e., it preserves the supremum of nonempty chains (or nonempty directed subsets) in either argument:

(supX)y=sup(Xy) y(supX) =sup(yX),

1Unlike at some other places, we do not require thatx≤yiff there is somezwithx+z=y.

for all nonempty chains (or directed sets) X ⊆S, where Xy= {xy ∶ x∈X} and yX is defined in the same way. It follows that product distributes over all sums:

(∑

i∈I

x_i)y= ∑

i∈I

x_iy y(∑

i∈I

x_i) = ∑

i∈I

yx_i,

wheneverxi∈S for alli∈I.

Suppose thatSis a continuous semiring andIis a set. Then thematrix semiring S^I×I,equipped with the pointwise sum operation, the usual matrix product opera- tion and the pointwise ordering is also a continuous semiring. The matrix product is meaningful since the sum of anyI-indexed family of elements ofS exists.

Continuous semirings are also closed under the formation ofpower series semir- ings. Suppose thatSis a continuous semiring andX is a set. As usual, letS⟪X^∗⟫ denote the semiring of all power seriess= ∑u∈X^∗⟨s, u⟩uoverX with coefficients in S. Each series smay be viewed as a functionX^∗→S mapping a wordu∈X^∗ to

⟨s, u⟩. Equipped with the pointwise order relation s≤s^′ iff ⟨s, u⟩ ≤ ⟨s^′, u⟩ for all u∈ X^∗, S⟪X^∗⟫ is a continuous semiring. The sum of any family of series is the pointwise sum.

Theorem 2.1. Suppose thatS is a continuous semiring. Then for each setX, the continuous semiring S⟪X^∗⟫ has the following universal property. Given a contin- uous semiringS^′, a continuous semiring morphismhS ∶S →S^′ and any function hX ∶ X → S^′ such that the elements of hS(S) commute with the elements of S^′, there is a unique continuous semiring morphismh^♯∶S⟪X^∗⟫ →S^′extendinghS and h_X.

Proof. We provide an outline of the proof. For details, see [7].

First we extend hX to a monoid morphism X^∗ → S^′, denoted just h. Then, for a series s∈S⟪X^∗⟫, we define h^♯(s) = ∑^u∈X^∗hS(⟨s, u⟩)h(u). It is clear that h^♯ extends hS and hX and preserves the constants 0 and 1. Then we prove that h^♯ is continuous and preserves the binary sum operation. It then follows thath^♯ preserves all finite and infinite sums. Last, we prove thath^♯preserves the product operation. Since the definition ofh^♯ was forced, it is unique.

3 Continuous semiring-semimodule pairs

Suppose now that S is a semiring and V is a commutative monoid as above. We callV a (left)S-semimodule[9] if there is an actionS×V →Ssubject to the usual

associativity, distributivity and unitary conditions:

(s1s2)v=s1(s2v) (s₁+s₂)v=s₁v+s₂v s(v1+v2) =sv1+sv2

1v=v 0v=0 s0=0

for alls, s1, s2∈S andv, v1, v2∈V. WhenV is anS-semimodule, we also say that (S, V)is asemiring-semimodulepair.

We call a semiring-semimodule pair(S, V)continuous ifSis a continuous semir- ing andV is a continuous commutative monoid such that the action is continuous in either argument:

(supX)v=sup(Xv) x(supY) =sup(xY)

for allx∈S, y ∈V and nonempty chains (or nonempty directed sets) X ⊆S and Y ⊆V. Of course,Xv= {sv∶ s∈X}andxY = {xw∶ w∈Y}. It follows that action distributes over summation on either side:

(∑

i∈I

x_i)v = ∑

i∈I

x_iv

x(∑

i∈I

v_i) = ∑

i∈I

xv_i for allx, xi∈S, v, vi∈V, i∈I, whereI is any index set.

Moreover, we call a continuous semiring-semimodule pair (S, V)a continuous ω-semiring-semimodule pair if it is equipped with an infinite product operation

∏n>1x_n mapping an ω-sequence (or ω-word) x₁x₂⋯ ∈ S^ω to ∏n≥1x_n ∈ V. The infinite product is subject to the following axioms:

Ax1

x∏

n≥1

x_n = ∏

n≥1

y_n, wherey1=xandyn+1=xn for alln≥1.

Ax2

∏

n≥1

xn = ∏

n≥1

xi_n⋯xi_n+1−1

where the sequence i1=1≤i2≤ ⋯increases without a bound and the product of an empty family is 1.

Ax3

∏

n≥1

(xn+yn) = ∑

z_n=x_norz_n=y_n

∏

n≥1

zn

Ax4

∏

n≥1

supX_n= sup

x_n∈X_n∏

n≥1

x_n

where for eachn, Xn⊆S is a nonempty chain (or a nonempty directed set).

It follows that

∏

n≥1

∑

i_n∈I_n

xi_n = ∑

i₁∈I₁,i₂∈I₂,...

∏

n≥1

xi_n (1)

where{x_in ∶ i_n∈I_n} is a family of elements ofS for alln≥1. Indeed,

∏

n≥1

∑

i_n∈I_n

xi_n = ∏

n≥1

sup

F_n⊆I_n{ ∑

i_n∈F_n

xi_n}

= sup

Fn⊆In

∏

n≥1{ ∑

in∈Fn

xi_n}

= sup

Fn⊆In

∑

in∈Fn

∏

n≥1

xi_n

= sup

F⊆I₁×I₂×... ∑

(i1,i2,...)∈F

∏

n≥1

x_in

= ∑

(i1,i2,...)∈I1×I2×...

∏

n≥1

x_in.

In particular, note that∏n≥1x_n=0 whenever there is some msuch thatx_mis 0. (This also follows from Ax1.) Moreover, infinite product is monotonic: ifxn≤yn

inS for alln≥1, then∏n≥1xn≤ ∏ⁿ≥1yn.

Suppose thatSis a continuous semiring. Then we define a star operationS→S as usual: s^∗= ∑n≥0sⁿ for alls∈S. It is known thats^∗ is the least solution of the fixed point equationx=sx+1 (and also ofx=xs+1) overS. And if (S, V)is a continuousω-semiring-semimodule pair, we define an omega operation S^ω→V by s^ω = ∏ⁿ≥1s for all s∈S. It is known that for each s∈ S, s^ω is a solution of the equationv=sv overV.

Complete equational and quasi-equational axiomatization of the equational prop- erties of the star operation in continuous semirings has been given in [2]. Among the identities satisfied by continuous semirings are the sum star and product star identities [3, 1]:

(x+y)^∗= (x^∗y)^∗x^∗ (xy)^∗=1+x(yx)^∗y Also 0^∗=1 and 1^∗=1^∗∗ hold.

When(S, V)is a continuousω-semiring-semimodule pair, the omega operation satisfies the sum omega and product omega identities [1]:

(x+y)^ω= (x^∗y)^ω+ (x^∗y)^∗x^ω (xy)^ω=x(yx)^ω

for allx, y∈S. Also 0^ω=0.

4 Matrices and series

4.1 Matrices

Suppose that (S, V) is a continuous ω-semiring-semimodule pair and I is a set.

Then, as mentioned above,S^I×I is a continuous semiring, and V^I is a continuous commutative monoid. There is a natural left action ofS^I×I onV^I defined similarly to matrix multiplication:

(M N)i = ∑

j∈I

Mi,jNj, i∈I,

for allM ∈S^I×I andN∈S^I. Moreover, we may define an infinite product operation by

(∏

n≥1

M_n)i = ∑

i=i1,i2,...

∏

n≥1

(M_n)in,in+1, i∈I.

Proposition 4.1. Suppose that(S, V)is a continuousω-semiring-semimodule pair and I is a set. Then, equipped with the pointwise orderings, (S^I×I, V^I) is also a continuousω-semiring-semimodule pair.

Proof. The fact thatS^I×I is a continuous semiring is proved in [7]. It is clear thatV^I is a continuous commutative monoid and that equipped with the action,(S^I×I, V^I) is a semiring-semimodule pair. The action is continuous in either argument, so that (S^I×I, V^I)is also a continuous semiring-semimodule pair. This can be proved by an argument similar to that used in [7] to establish that product inS^I×I is continuous in either argument.

In order to conclude, we still need to prove that the infinite product over matri- ces satisfies the axioms Ax1-Ax4. LetMn∈S^I×I for alln≥1, and defineM^′=M1

andM_n^′ =Mn+1 for alln≥1. Then for everyi∈I, the ith component of∏n≥1M_n^′ is

∑

j∈I

(M₁)i,j ∑

j=j₁,j₂,...∈I(∏

n≥1

(M_n+1)jn,jn+1) = ∑

i=i₁,i₂,...∈I

∏

n≥1

(M_n)in,in+1

which is theith component of∏n≥1Mn. Hence, Ax1 holds.

Suppose now that the sequence 1=k1≤k2≤ ⋯increases without a bound and defineM_n^′ =Mi_n⋯Mi_n+1−1 for alln≥1. Then for eachi∈I, thejth component of

∏n≥1M_n^′ is

∑

j=j1,j2,...∈I

∏

n≥1

(M^′)j_n,j_n+1

= ∑

j=j1,j2,...∈I

∏

n≥1

∑

jn=`1,`2,...,`_in+1−in=jn+1

(Mi_n)`<sub>1</sub>,`₂⋯(Mi_n+1−1)`<sub>in+1</sub>−in−1,`_in+1−in

= ∑

j=j₁,j₂,...

∏

n≥1

(Mj)j_n,j_n+1,

proving Ax2.

In order to prove that Ax3 holds, let Mn, M_n^′ ∈ S^I×I for all n≥ 1. Then for everyi∈I,theith component of∏n≥1(Mn+M_n^′)is

∑

i=i1,i2,...

∏

n≥1

(Mn+M_n^′)i_n,i_n+1= ∑

Pn=Mn orPn=M_n^′

∑

i=i1,i2,...

∏

n≥1

Pi_n,i_n+1

= ( ∑

Pn=MnorPn=M_n^′

∏

n≥1

Pn)i,

i.e., theith component∑Pn=MnorPn=M_n^′ ∏n≥1Pn. This proves Ax3.

Suppose now that for eachn≥1,Mn is a nonempty chain w.r.t. the pointwise ordering of matrices inS^I×I. We want to prove that

∏

n≥1

supMn = sup

M_n∈Mn

∏

n≥1

M_n. (2)

Leti∈Ibe fixed. Then theith component of∏n≥1supMn is

∑

i=i1,i2,...

∏

n≥1

(supM)in,in+1 = ∑

i=i1,i2,...

sup

Mn∈Mn

∏

n≥1

(M_n)in,in+1

= sup

M_n∈Mn

∑

i=i₁,i₂,...

∏

n≥1

(Mn)i_n,i_n+1

= sup

M_n∈Mn

(∏

n≥1

M_n)i.

Here, we used the fact, proved in [7], that summation is continuous. It follows that (2) holds.

Hence, if (S, V) is a continuous ω-semiring-semimodule pair, then (S^I×I, V^I) comes with a star operation and an omega operation.

4.2 Series

We may also construct continuousω-semiring-semimodule pairs of power series. To this end, we assume that S is a continuous ω-semiring equipped with an infinite productS^ω→Ssubject to axioms similar to those defining continuousω-semiring- semimodule pairs. This amounts to requiring that, equipped with left multiplication as the action and the infinite product,(S, S)is a continuousω-semiring-semimodule pair.

LetX be any set. We already know that S⟪X^∗⟫ is a continuous semiring. In a similar way,S⟪X^ω⟫, equipped with the pointwise sum operation and pointwise ordering, is a continuous commutative monoid, and the action ofS⟪X^∗⟫onS⟪X^ω⟫ defined by

sr = ∑

w∈A^ω

∑

w=uv

⟨s, u⟩⟨r, v⟩uv

turns S⟪X^ω⟫ into an S⟪X^∗⟫-semimodule. Moreover, the action is continuous in either argument and it is easy to check that Ax1-Ax4 hold.

Letsn∈S⟪X^∗⟫for alln≥1. We definer= ∏ⁿ≥1sn∈S⟪X^ω⟫as follows. Given v∈X^ω,we define

⟨r, v⟩ = ∑

v=v₁v₂...

∏

n≥1

⟨sn, vn⟩

Proposition 4.2. Let S be a continuous ω-semiring and X be a set.

Then(S⟪X^∗⟫, S⟪X^ω⟫)is a continuous ω-semiring-semimodule pair.

Proof. First we establish Ax1. Lets∈S⟪X^∗⟫andsn∈S⟪X^∗⟫for alln≥1. Then, for allw∈X^ω,

⟨s∏

n≥1

sn, w⟩ = ∑

w=uv

⟨s, u⟩⟨∏

n≥1

sn, v⟩

= ∑

w=uv

⟨s, u⟩ ∑

v=v1v2...

∏

n≥1

⟨sn, vn⟩

= ∑

w=uv₁v₂...

⟨s, u⟩ ∏

n≥1

⟨s_n, v_n⟩

= ∑

w=v1v2...

⟨s^′_n, vn⟩

= ⟨∏

n≥1

s^′_n, w⟩,

wheres^′₁=sands^′_n+1=s_n for alln≥1.

Let again s_n∈S⟪X^∗⟫ for alln≥1. Suppose that the sequencei₁ =1≤i₂≤ ⋯ increases without a bound. For eachn≥1, define s^′_n =s_in⋯s_in+1−1. Then for all w∈X^ω,

⟨∏

n≥1

s_n, w⟩ = ∑

w=v₁v₂...∏

n≥1

⟨s_n, v_n⟩

= ∑

w=v₁v₂...

∏

n≥1

⟨si_n, vi_n⟩⋯⟨si_n+1−1, vi_n+1−1⟩

= ∑

w=u₁u₂⋯∏

n≥1

⟨s^′_n, u_n⟩

= ⟨∏

n≥1

s^′_n, w⟩,

proving Ax2.

Next, suppose that sn, s^′_n∈S⟪X^∗⟫for alln≥1. Then for allw∈X^ω,

⟨∏

n≥1

(sn+s^′_n), w⟩ = ∑

w=v₁v₂...

∏

n≥1

⟨sn+s^′_n, vn⟩

= ∑

w=v1v2⋯ ∑

r_n=s_norr_n=s^′_n

∏

n≥1

⟨rn, vn⟩

= ∑

rn=snorrn=s^′_n

⟨∏

n≥1

r_n, w⟩,

proving Ax3.

Finally, suppose that (In,≤) is a nonempty directed partially ordered set for eachn≥1 and si∈S⟪X^∗⟫ for all i∈In, n≥1 such that si≤sj wheneveri≤j in In. We want to prove that

∏

n≥1

sup

i∈In

si= sup

i1∈I1,i2∈I2,...∏

n≥1

si_n. Letw∈X^ω. Then

⟨∏

n≥1

sup

i∈I_n

s_i, w⟩ = ∑

w=v₁v₂...∏

n≥1

⟨sup

i∈I_n

s_i, v_n⟩

= ∑

w=v₁v₂...

∏

n≥1

sup

i∈In

⟨si, vn⟩

= ∑

w=v₁v₂...

sup

i₁∈I₁,i₂∈I₂,...∏

n≥1

⟨s_in, v_n⟩

= sup

i1∈I1,i2∈I2,... ∑

w=v₁v₂...

∏

n≥1

⟨si_n, vn⟩

= sup

i₁∈I₁,i₂∈I₂,...⟨∏

n≥1

sin, w⟩

= ⟨ sup

i₁∈I₁,i₂∈I₂,...∏

n≥1

s_in, w⟩, proving Ax4.

Hence, ifS is a continuous semiring, then for any setX,(S⟪X^∗⟫, S⟪X^ω⟫)has a star and an omega operation.

5 Freeness

Suppose now that(S, V)and(S^′, V^′)are continuousω-semiring-semimodule pairs.

We say that a pair of functionsh= (h_S, h_V)withh_S : S→S^′andh_V : V →V^′is a continuousω-semiring-semimodule pair morphism ifh_S is a continuous semiring homomorphism, h_V is a continuous monoid homomorphism, h_S and h_V jointly preserve the action, moreover, infinite product is preserved:

h_V(∏

n≥1

x_n) = ∏

n≥1

h_S(x_n) for allxn∈S, n≥1. In this section we prove:

Theorem 5.1. Suppose that S is a continuous ω-semiring. Then for each set X, the continuousω-semiring-semimodule pair(S⟪X^∗⟫, S⟪X^ω⟫)has the following universal property. Let (S^′, V^′) be a continuousω-semiring-semimodule pair,hS ∶ S→S^′a continuous semiring morphism andhX ∶X→S^′a function. Suppose that the elements of hS(S)commute with the elements of S^′ and the following infinite commutativity holds:

∏

n≥1

s_ns^′_n= (∏

n≥1

s_n)(∏

n≥1

s^′_n)

for all sn ∈ hS(S) and s^′_n ∈ S^′. Then there is a unique continuous semiring- semimodule morphismh^♯= (h^♯_S, h^♯_V)extendinghS andhX.

Proof. We have already shown that (S⟪X^∗⟫, S⟪X^ω⟫) is a continuous semiring- semimodule pair. Let us first extendhto a functionX^∗→S, denoted justh, so that it becomes a (multiplicative) monoid homomorphism. Then leth^♯_S : S⟪X^∗⟫ →S^′ be defined by

h^♯_S(s) = ∑

u∈X^∗

h_S(⟨s, u⟩)h(u). It is known thath^♯_S is a continuous semiring homomorphism.

Next we extendhto a functionX^ω→V^′by definingh(v) = ∏ⁱ≥1hX(xi)for each v=x1x2. . .inX^ω. Finally, whens∈S⟪X^ω⟫, leth^♯_V(s) = ∑^v∈X^ωhS(⟨s, v⟩)h(v).

Suppose that s_i∈S⟪X^ω⟫ for all i∈I, where I is nonempty directed partially ordered set, ordered by the relation ≤. Moreover, suppose that si ≤sj whenever i≤j in I and lets=sup_i∈Isi. Then

h^♯_V(s) = ∑

v∈X^ω

hS(⟨s, v⟩)h(v)

= ∑

v∈X^ω

sup

i∈I

hS(⟨si, v⟩)h(v)

=sup

i∈I ∑

v∈X^ω

h_S(⟨s_i, v⟩)h(v)

=sup

i∈I

h^♯_V(si),

proving thath^♯_V is continuous. To prove thath^♯_V preserves the sum operation, let s₁, s₂∈S⟪X^ω⟫. Then

h^♯_V(s₁+s₂) = ∑

v∈X^ω

h_S(⟨s₁+s₂, v⟩)h(v)

= ∑

v∈X^ω

hS(⟨s1, v⟩)h(v) +hS(⟨s2, v⟩)h(v)

= ∑

v∈X^ω

hS(⟨s1, v⟩)h(v) + ∑

v∈X^ω

hS(⟨s2, v⟩)h(v)

=h^♯_V(s₁) +h^♯_V(s₂).

It is clear thath^♯_V preserves 0. In order to prove thath^♯_S andh^♯_V jointly preserve the action, lets∈S⟪X^∗⟫andr∈S⟪X^ω⟫. Then

h^♯_V(sr) = ∑

v∈X^ω

hS(⟨sr, v⟩)h(v)

= ∑

v∈X^ω

∑

v=uw

h_S(⟨s, u⟩)h_S(⟨s, w⟩)h(u)h(w)

= ∑

v∈X^ω

∑

v=uw

h_S(⟨s, u⟩)h(u)h_S(⟨r, w⟩)h(w)

= ∑

u∈X^∗

hS(⟨s, u⟩)h(u) ∑

w∈X^ω

hS(⟨r, w⟩)h(w)

=h^♯_S(s)h^♯_V(r).

Finally, we prove that h^♯_V preserves the infinite product. To this end, letsn ∈ S⟪X^∗⟫for alln≥1. We want to prove that h^♯_V(∏ⁿ≥1sn) = ∏ⁿ≥1h^♯_S(sn).

h^♯_V(∏

n≥1

sn) = ∑

v∈X^ω

hS(⟨∏

n≥1

sn, v⟩)h(v)

= ∑

v∈X^ω

∑

v=v₁v₂...∏

n≥1

h_S(⟨s_n, v_n⟩)h(v_n)

= ∑

v∈X^ω

∑

v=v₁v₂...

∏

n≥1

hS(⟨sn, vn⟩) ∏

n≥1

h(vn)

= ∏

n≥1( ∑

v_n∈X^∗

h_S(⟨s_n, v_n⟩)h(v_n))

= ∏

n≥1

hS(sn).

It is clear that h_S extends h. Since the definitions of h_S and h_V were forced, they are unique.

6 The category Matr

All categoriesC in the paper will have sets as objects. The composition of mor- phismsf : X →Y andg∶Y →Z will be denotedf○g. We usually letid_X denote the identity morphismX→X.

Our categories will have finite coproducts. For a sequenceX1, . . . , Xnof objects, the coproductX1⊕⋯⊕Xnwill be given by disjoint unionX1⊎⋯⊎Xn. In particular, the empty set∅will serve as initial object.

Let X1, . . . , Xn be objects. For each i = 1, . . . , n, the ith coproduct injection inX_i : Xi→X1⊕ ⋯ ⊕Xn will always be determined by the embedding ofXi into X1⊎ ⋯ ⊎Xn. We will let !X denote the unique morphism ∅ →X. Moreover, if fi∶Xi→Xfori=1, . . . , n, then we will let⟨f1, . . . , fn⟩denote the unique morphism f : X₁⊕ ⋯ ⊕X_n →X with in_Xi○f =f_i for all i. And when f_i∶X_i→Y_i, where i∈ {1, . . . , n}, then we letf₁⊕⋯⊕f_ndenote the unique morphismf ∶X₁⊕⋯⊕X_n→ Y₁⊕ ⋯ ⊕Y_n within_Xi○f =f_i○in_Yi for alli.

For anyX, Y, the hom-setC(X, Y)of morphismsX →Y will be both a complete partial order (C,≤) and a commutative monoid (C(X, Y),+,0X,Y) such that the zero morphism 0X,Y is also least w.r.t. ≤and the operation+is continuous in both of its arguments. Also, the operation of composition will be continuous in both arguments. Moreover, the partial order will be related to the coproduct structure so that for anyf, g: X1⊕ ⋯ ⊕Xn→Y,f ≤g iffinX_i○f ≤nX_i○g. It follows that whenfi∶Xi→Yi, wherei∈ {1, . . . , n}, thenf1⊕ ⋯ ⊕fn≤g1⊕ ⋯ ⊕gn ifffi≤gi for alli.

The following identities will hold for all f, g∶X →Y andh∶Y →Z:

(f+g) ○h=f○h+g○h 0X,Y ○h=0X,Z

Finally, our categories will be equipped with a dagger operation mapping a morphismf : X→X⊕Y to a morphismf^† : X→Y. This operation will always be a fixed point operation, so that the following fixed point identity will hold:

f^† = f○ ⟨f^†,idY⟩ for allf : X→X⊕Y.

Iteration categories are categories with finite coproducts and a dagger operation satisfying certain identities including the above fixed point identity, the parameter identity

(f○ (idX⊕g))^† = f^†○g

wheref ∶X→X⊕Y andg∶Y →Z, the double dagger identity f^†† = (f○ (⟨idX,idX⟩ ⊕idY))^†,

wheref ∶X →X⊕X⊕Y, to name a few, and some other identities including the group identities that we will described later. All of our categories will be iteration categories.

Suppose now that (S, V) is a continuous ω-semiring-semimodule pair. Then (S, V)determines a categoryMatr_(S,V)whose objects are all sets and a morphism I→J is an ordered pair (A, u), where A∈S^I×J and u∈V^I. Hence a morphism I→I is an element of the semiring-semimodule pair(S^I×I, V^I).

Composition is defined as follows. Suppose that(A, u) ∶I→J and(B, v) ∶J → K. Then we define (A, u) ○ (B, v) = (AB, u+Av) ∶ I → K. It is easy to check that composition is associative with the morphisms(E^I×I, 0^I) ∶I→ I serving as identities, where E^I×I is the unit matrix inS^I×I and 0^I denotes the zero element ofV^I. (For finite sets, this category is defined in [1].)

The partial order ≤ on a hom-set of Matr_(S,V) is defined pointwise, so that when (A, u),(B, v) ∶I →J, then (A, u) ≤ (B, v)iff A_i,j ≤B_i,j and u_i ≤v_i for all i∈I andj∈J. Clearly, each hom-set forms a complete partial order, and it is not difficult to verify that composition is continuous.

We can also impose a commutative monoid structure on the hom-sets by defining (A, u)+(B, v)pointwise, for all(A, u),(B, v) ∶I→J. Hence(A, u)+(B, v) = (C, w) withC_i,j =A_i,j+B_i,j andw_i=u_i+v_i for alli∈I and j∈J. The zero morphism I→J is the morphism(0^I×J,0^I)consisting of two zero matrices. We denote it by 0I,J, or just 0. We have

((A, u) + (B, v)) ○ (C, w) = (A, u) ○ (C, w) + (B, v) ○ (C, w) 0I,j○ (C, w) =0I,K

for all(C, w): J→K. It is not difficult to prove that composition is continuous.

Coproduct is given by disjoint union on objects. WhenX1, . . . , Xn is a sequence of sets andi∈ {1, . . . , n}, thenith coproduct embeddinginiconsists of anXi×(X1⊎

⋯ ⊎Xn)matrix whosexi×Xi submatrix is an identity matrix and whoseXi×Xj

matrices are all zero matrices forj≠i, together with the column matrix 0^Xi∈V^Xi.

We have already noted that for each setI,(S^I×I, V^I)is a continuousω-semiring- semimodule pair. Hence it comes with a star and an omega operation: For each A∈S^I×I, A^∗ = ∑ⁿ≥0Aⁿ ∈ S^I×I and A^ω = ∏ⁿ≥1A in V^I. These operations satisfy the identities mentioned above. And in fact,

(A+B)^∗= (A^∗B)^∗A^∗, A, B∈S^I×I

(AB)^∗=EI+A(BA)^∗B, A∈S^I×J, B∈S^J×I and

(A+B)^ω= (A^∗B)^∗A^ω+ (A^∗B)^ω, A, B∈S^I×I (AB)^ω=A(BA)^ω, A∈S^I×J, B∈S^J×I Suppose now thatI=J⊎K andM∈S^I×I is partitioned as

M = ( a b c d ). Then

M^∗= ( (a+bd^∗c)^∗ (a+bd^∗c)^∗bd^∗ (d+ca^∗b)^∗ca^∗ (d+ca^∗b)^∗ ) and

M^ω= ( (a+bd^∗c)^ω+ (a+bd^∗c)^∗bd^ω (d+ca^∗b)^ω+ (d+ca^∗b)^∗ca^ω )

The star and omega operations together give rise to a dagger operation overMatr_(S,V) that map a morphism X → X ⊕Y to a morphism X → Y. To define it, let (A, u) ∶X → I⊕J, and partition A as (a, b) with a∈ S^I×I and b∈ S^I×J. Then we define (A, u)^† = (a^∗b, a^ω+a^∗v) ∶I → J. Clearly, (A, u)^† is a solution of the equation

(X, x) = (A, u) ( (X, x)

(EJ,0) ) = (aX+b, ax+u)

where (X, x) ranges over the morphisms I →J. It is known that equipped with dagger,Matr₍S,V) is an iteration category.

7 Categories of Series

7.1 The category Ser

Suppose thatSis a commutative continuous semiring and Σ is a set. We define the category SerS,Σ whose objects are all sets and a morphism X →Y is a function f ∶X→S⟪(Y ⊎Σ)^∗⟫, or alternatively, a tuple(fx)x∈X of seriesfx∈S⟪(Y ⊎Σ)^∗⟫. Suppose that f ∶ X → Y and g ∶ Y → Z. Then we define f○g as the function composition off andg^♯, the extension of the functionY⊎Σ toS⟪(Z⊎Σ)^∗⟫which

agrees with g on Y and is the identity function on Σ to a continuous semiring homomorphismS⟪(Y ⊎Σ)^∗⟫ →S⟪(Z⊎Σ)^∗⟫. For each X, the identity morphism idX is the embedding of X into X⊎Σ.

Each hom-set of morphisms X → Y of the category SerS,Σ has the structure of a complete partial order and commutative monoid. For any f, g∶ X → Y, we define f ≤ g iff fx ≤gx for allx, and similarly, (f+g)x =fx+gx for all x. The neutral element is the series 0X,Y whose components are all 0. This is also the least morphism X → Y. Composition of morphisms is continuous as is the sum operation.

Also, Ser_S has finite coproducts are given by disjoint sum on objects. The coproductX₁⊕ ⋯ ⊕X_n of a sequenceX₁, . . . , X_n of sets is given by disjoint union, and for eachi∈ {1, . . . , n}, in_Xi ∶X_i →X₁⊕ ⋯ ⊕X_n is the embedding of X_i into S⟪(X₁⊎ ⋯ ⊎X_n⊎Σ)^∗⟫.

We define a dagger operation onSer_S,Σwhich maps a morphismf∶X →X⊕Y to f^†∶X → Y. The morphismf^† is given as the least solution of the fixed point equation

ξ=f○ ⟨ξ,idY⟩.

In more detail,f^† =sup_n≥0f⁽ⁿ⁾, where f⁽⁰⁾=0 and f⁽ⁿ⁺¹⁾=f○ ⟨f⁽ⁿ⁾,id_Y⟩. It is known that equipped with this dagger operation,SerS,Σis an iteration category.

7.2 The category Ser

.

Suppose now thatSis a commutative continuous ω-semiring satisfying the infinite commutativity identity. Then we define another categorySer^ω_S,Σwith sets as ob- jects as above. However, a morphismf ∶X→Y is now a triplet(f0, fM, fω)with f0∶X→Y inSerS,Σand(fM, fω) ∶X→Y inMatr_{(S⟪(Y⊎Σ)}^∗_{⟫,S⟪(Y⊎Σ)}ω⟫). Hence, f₀∶X→S⟪(Y ⊎Σ)^∗⟫, f_M ∈S⟪(Y ⊎Σ)^∗⟫^X×Y andf_ω∶X→S⟪(Y ⊎Σ)^ω⟫.

Composition is defined as follows. Let f ∶ X →Y and g ∶Y → Z. Then the components ofh=f○g∶X→Z are given by

• h₀=f₀○g₀, where the composition is taken fromSer_S,Σ, and

• (h_M, h_ω) =g₀^♯((f_M, f_ω)) ○ (g_M, g_ω) = (g^♯₀(f_M), g^♯₀(f_ω)) ○ (g_M, g_ω)where the composition is taken from the categoryMatr_{(S⟪(Z⊎Σ)}∗⟫,S⟪(Z⊎Σ)^ω⟫).

Note that the definition is legitimate, since(S⟪(Z⊎Σ)^∗⟫, S⟪(Z⊎Σ)^ω⟫)is a con- tinuousω-semiring-semimodule pair andg^♯₀((f_M, f_ω)) = (g₀^♯(f_M), g₀^♯(f_ω))is a mor- phismX →Y and(g_M, g_ω)is a morphismY →ZinMatr_{(S⟪(Z⊎Σ)}∗⟫,S⟪(Z⊎Σ)^ω⟫). Of course,g^♯₀is the extension ofg₀ to a continuousω-semiring-semimodule morphism andg₀^♯(fM)andg₀^♯(fω)are formed component-wise. It is a routine matter to verify that composition is associative. The identity morphism X →X is determined by the corresponding identity morphisms inSerS,Σ andMatr_{(S⟪(X⊎Σ)}∗⟫,S⟪(X⊎Σ)^ω⟫).

Each hom-set of morphismsX→Y is partially ordered by the component-wise order inherited fromSer_(S,Σ)andMatr_{(S⟪(Y⊎Σ)}∗⟫,S⟪(Y⊎Σ)^ω⟫). Also, each hom-set