List of rules of inference

28 year-old Painting Investments Worker Truman from Regina, usually spends time with pastimes for instance interior design, property developers in new launch ec Singapore and writing. Last month just traveled to City of the Renaissance. In category theory, a branch of mathematics, a monad, or triple is an (endo-)functor, together with two natural transformations. Monads are used in the theory of pairs of adjoint functors, and they generalize closure operators on partially ordered sets to arbitrary categories. Saunders Mac Lane adopted the philosophical term "monad" (a single entity that generates all other entities) for this construct, noting the construct's ability to generate a corresponding category.

Introduction

If $F$ and $G$ are a pair of adjoint functors, with $F$ left adjoint to $G$, then the composition $G\circ F$ is a monad. Therefore, a monad is an endofunctor. If $F$ and $G$ are inverse functors, the corresponding monad is the identity functor. In general, adjunctions are not equivalences — they relate categories of different natures. The monad theory matters as part of the effort to capture what it is that adjunctions 'preserve'. The other half of the theory, of what can be learned likewise from consideration of $F\circ G$, is discussed under the dual theory of comonads.

The monad axioms can be seen at work in a simple example: let $G$ be the forgetful functor from the category Grp of groups to the category Set of sets. Then as $F$ we can take the free group functor.

This means that the monad

$T=G\circ F$

takes a set $X$ and returns the underlying set of the free group $\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{e}(X)$. In this situation, we are given two natural morphisms:

$X\to T(X)$

by including any set $X$ in $\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{e}(X)$ in the natural way, as strings of length 1. Further,

$T(T(X))\to T(X)$

can be made out of a natural concatenation or 'flattening' of 'strings of strings'. This amounts to two natural transformations

$I\to T$

and

$T\circ T\to T$

They will satisfy some axioms about identity and associativity that result from the adjunction properties.

Those axioms are formally similar to the monoid axioms. They are taken as the definition of a general monad (not assumed a priori to be connected to an adjunction) on a category.

If we specialize to categories arising from partially ordered sets $(P,\le )$ (with a single morphism from $x$ to $y$ iff $x\le y$), then the formalism becomes much simpler: adjoint pairs are Galois connections and monads are closure operators.

Every monad arises from some adjunction, in fact typically from many adjunctions. Two constructions introduced below, the Kleisli category and the category of Eilenberg-Moore algebras, are extremal solutions of the problem of constructing an adjunction that gives rise to a given monad.

The example about free groups given above can be generalized to any type of algebra in the sense of a variety of algebras in universal algebra. Thus, every such type of algebra gives rise to a monad on the category of sets. Importantly, the algebra type can be recovered from the monad (as the category of Eilenberg-Moore algebras), so monads can also be seen as generalizing universal algebras. Even more generally, any adjunction is said to be monadic (or tripleable) if it shares this property of being (equivalent to) the Eilenberg-Moore category of its associated monad. Consequently Beck's monadicity theorem, which gives a criterion for monadicity, can be used to show that an arbitrary adjunction can be treated as a category of algebras in this way.

The notion of monad was invented by Roger Godement in 1958 under the name "standard construction." In the 1960s and 1970s, many people used the name "triple." The now standard term "monad" is due to Mac Lane.

Formal definition

If $C$ is a category, a monad on $C$ consists of a functor $T:C\to C$ together with two natural transformations: $\eta :1_C\to T$ (where $1_C$ denotes the identity functor on $C$) and $\mu :T^2\to T$ (where $T^2$ is the functor $T\circ T$ from $C$ to $C$). These are required to fulfill the following conditions (sometimes called coherence conditions):

• $\mu \circ T\mu =\mu \circ \mu T$ (as natural transformations $T^3\to T$);
• $\mu \circ T\eta =\mu \circ \eta T=1_T$ (as natural transformations $T\to T$; here $1_T$ denotes the identity transformation from $T$ to $T$).

We can rewrite these conditions using following commutative diagrams:

File:Monad mult.png

File:Monad unit1.png

See the article on natural transformations for the explanation of the notations $T\mu$ and $\mu T$, or see below the commutative diagrams not using these notions:

The first axiom is akin to the associativity in monoids, the second axiom to the existence of an identity element. Indeed, a monad on $C$ can alternatively be defined as a monoid in the category ${𝐄𝐧𝐝}_C$ whose objects are the endofunctors of $C$ and whose morphisms are the natural transformations between them, with the monoidal structure induced by the composition of endofunctors.

Comonads and their importance

The categorical dual definition is a formal definition of a comonad (or cotriple); this can be said quickly in the terms that a comonad for a category $C$ is a monad for the opposite category $C^{\mathrm{o}\mathrm{p}}$. It is therefore a functor $U$ from $C$ to itself, with a set of axioms for counit and comultiplication that come from reversing the arrows everywhere in the definition just given.

Since a comonoid is not a basic structure in abstract algebra, this is less familiar at an immediate level.

The importance of the definition comes in a class of theorems from the categorical (and algebraic geometry) theory of descent. What was realised in the period 1960 to 1970 is that recognising the categories of coalgebras for a comonad was an important tool of category theory (particularly topos theory). The results involved are based on Beck's theorem. Roughly what goes on is this: while it is simple set theory that a surjective mapping of sets is as good as the imposition of the equivalence relation 'in the same fiber', for geometric morphisms what you should do is pass to such a coalgebra subcategory.

Examples

The rich set of examples is given by adjunctions (see Monads and adjunctions), and the free group example mentioned above belongs to that set. Another example, on the category $𝐒𝐞𝐭$: for a set $A$ let $T(A)$ be the power set of $A$ and for a function $f:A\to B$ let $T(f)$ be the function between the power sets induced by taking direct images under $f$. For every set $A$, we have a map ${\eta }_A:A\to T(A)$, which assigns to every $a\in A$ the singleton $\{a\}$. The function

${\mu }_A:T(T(A))\to T(A)$

in fact is union (arbitrary union, not finitary union). These data describe a monad.

Closure operators are monads on preorder categories.

Algebras for a monad

DTZ's public sale group in Singapore auctions all forms of residential, workplace and retail properties, outlets, homes, lodges, boarding homes, industrial buildings and development websites. Auctions are at present held as soon as a month.

We will not only get you a property at a rock-backside price but also in an space that you've got longed for. You simply must chill out back after giving us the accountability. We will assure you 100% satisfaction. Since we now have been working in the Singapore actual property market for a very long time, we know the place you may get the best property at the right price. You will also be extremely benefited by choosing us, as we may even let you know about the precise time to invest in the Singapore actual property market.

The Hexacube is offering new ec launch singapore business property for sale Singapore investors want to contemplate. Residents of the realm will likely appreciate that they'll customize the business area that they wish to purchase as properly. This venture represents one of the crucial expansive buildings offered in Singapore up to now. Many investors will possible want to try how they will customise the property that they do determine to buy by means of here. This location has offered folks the prospect that they should understand extra about how this course of can work as well.

Singapore has been beckoning to traders ever since the value of properties in Singapore started sky rocketing just a few years again. Many businesses have their places of work in Singapore and prefer to own their own workplace area within the country once they decide to have a everlasting office. Rentals in Singapore in the corporate sector can make sense for some time until a business has discovered a agency footing. Finding Commercial Property Singapore takes a variety of time and effort but might be very rewarding in the long term.

is changing into a rising pattern among Singaporeans as the standard of living is increasing over time and more Singaporeans have abundance of capital to invest on properties. Investing in the personal properties in Singapore I would like to applaud you for arising with such a book which covers the secrets and techniques and tips of among the profitable Singapore property buyers. I believe many novice investors will profit quite a bit from studying and making use of some of the tips shared by the gurus." – Woo Chee Hoe Special bonus for consumers of Secrets of Singapore Property Gurus Actually, I can't consider one other resource on the market that teaches you all the points above about Singapore property at such a low value. Can you? Condominium For Sale (D09) – Yong An Park For Lease

In 12 months 2013, c ommercial retails, shoebox residences and mass market properties continued to be the celebrities of the property market. Models are snapped up in report time and at document breaking prices. Builders are having fun with overwhelming demand and patrons need more. We feel that these segments of the property market are booming is a repercussion of the property cooling measures no.6 and no. 7. With additional buyer's stamp responsibility imposed on residential properties, buyers change their focus to commercial and industrial properties. I imagine every property purchasers need their property funding to understand in value.

Suppose that $(T,\eta ,\mu )$ is a given monad on a category $C$.

A $T$-algebra $(x,h)$ is an object $x$ of $C$ together with an arrow $h:Tx\to x$ of $C$ called the structure map of the algebra such that the diagrams

File:Monad alg mult.png

and

File:Monad alg unit.png

commute.

A morphism $f:(x,h)\to (x^{\prime },h^{\prime })$ of $T$-algebras is an arrow $f:x\to x^{\prime }$ of $C$ such that the diagram

File:Monad alg morph.png

commutes.

The category $C^T$ of $T$-algebras and their morphisms is called the Eilenberg-Moore category or category of (Eilenberg-Moore) algebras of the monad $T$. The forgetful functor $C^T$→$C$ has a left adjoint $C$→$C^T$ taking $x$ to the free algebra $(Tx,{\mu }_x)$.

Given the monad $T$, there exists another "canonical" category $C_T$ called the Kleisli category of the monad $T$. This category is equivalent to the category of free algebras for the monad $T$, i. e. the full subcategory of $C^T$ whose objects are of the form $(Tx,{\mu }_x)$, for $x$ an object of $C$.

Monads and adjunctions

An adjunction $(F,G,\eta ,\epsilon )$ between two categories $C$ and $D$ (where $F:C\to D$ is left adjoint to $G:D\to C$ and $\eta$ and $\epsilon$ are respectively the unit and the counit) always defines a monad $(GF,\eta ,G\epsilon F)$.

Conversely, it is interesting to consider the adjunctions which define a given monad $(T,\eta ,\mu )$ this way. Let $𝐀𝐝𝐣(C,T)$ be the category whose objects are the adjunctions $(F,G,e,\epsilon )$ such that $(GF,e,G\epsilon F)=(T,\eta ,\mu )$ and whose arrows are the morphisms of adjunctions which are the identity on $C$. Then this category has

• an initial object $(F_T,G_T,\eta ,{\mu }_T):C\to C_T$, where $C_T$ is the Kleisli category,
• a terminal object $(F^T,G^T,\eta ,{\mu }^T):C\to C^T$, where $C^T$ is the Eilenberg-Moore category.

An adjunction $(F,G,\eta ,\epsilon )$ between two categories $C$ and $D$ is a monadic adjunction when the category $D$ is equivalent to the Eilenberg-Moore category $C^T$ for the monad $T=GF$. By extension, a functor $G:D\to C$ is said to be monadic if it has a left adjoint $F$ forming a monadic adjunction. Beck's monadicity theorem gives a characterization of monadic functors.

Uses

Monads are used in functional programming to express types of sequential computation (sometimes with side-effects). See monads in functional programming, and the more mathematically oriented Wikibook module b:Haskell/Category theory.

In categorical logic, an analogy has been drawn between the monad-comonad theory, and modal logic via closure operators, interior algebras, and their relation to models of S4 and Intuitionistic logics.

Generalization

It is possible to define monads in a 2-category $C$. Monads described above are monads for $C=𝐂𝐚𝐭$.

See also

• Distributive law between monads
• Strong monad
• Monad (disambiguation) for other meanings of the term.
• Polyad

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

Further reading

• Daniele Turi: Category Theory Lecture Notes (1996-2001), based on MacLane's book "Categories for the Working Mathematician"
• Michael Barr and Charles Wells: Category Theory for Computing Science (1999).
• Roger Godement: Topologie Algébrique et Théorie des Faisceaux. Actualités Sci. Ind. No. 1252. Publ. Math. Univ. Strasbourg. No. 13 Hermann, Paris 1958 viii+283 pp.
• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534

External links

• Monads, five short lectures (with one appendix).
• John Baez's This Week's Finds in Mathematical Physics (Week 89) covers monads in 2-categories.