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The letters id with an object subscript, the map that changes nothing.
Also known as: Identity map, id, Identity arrow
Every object in a category comes with an identity arrow to itself, and it is what makes composition into an algebra rather than a bare operation. Writing id_X rather than just id matters because the subscript is the only thing saying which object's identity is meant.
Denotes the identity morphism on a named object: the arrow that leaves every element where it is.
Contexts: Mathematics
Is required to exist for every object by the definition of a category, and is what makes an isomorphism definable as a pair of maps composing to identities.
Contexts: Mathematics
The two lowercase letters i and d with a capital letter set below and to the right.