Difference between revisions of "Injection"
From Maths
m (Added navbox... this page does need updating though) |
(Added arrow with tail notation) |
||
| Line 5: | Line 5: | ||
Or equivalently: | Or equivalently: | ||
* <math>\forall x_1,x_2\in X[x_1\ne x_2\implies f(x_1)\ne f(x_2)]</math> (the [[Contrapositive|contrapositive]] of the above) | * <math>\forall x_1,x_2\in X[x_1\ne x_2\implies f(x_1)\ne f(x_2)]</math> (the [[Contrapositive|contrapositive]] of the above) | ||
| + | Sometimes an injection is denoted {{M|\rightarrowtail}}{{rNOSTYM}} (and a [[surjection]] {{M|\twoheadrightarrow}} and a [[bijection]] is both of these combined (as if super-imposed on top of each other) - there is no LaTeX arrow for this however) - we do not use this convention. | ||
==Statements== | ==Statements== | ||
* [[Every injection yields a bijection onto its image]] | * [[Every injection yields a bijection onto its image]] | ||
Revision as of 00:06, 4 February 2017
An injective function is 1:1, but not nessasarally onto.
Definition
For a function [math]f:X\rightarrow Y[/math] every element of [math]X[/math] is mapped to an element of [math]Y[/math] and no two distinct things in [math]X[/math] are mapped to the same thing in [math]Y[/math]. That is[1]:
- [math]\forall x_1,x_2\in X[f(x_1)=f(x_2)\implies x_1=x_2][/math]
Or equivalently:
- [math]\forall x_1,x_2\in X[x_1\ne x_2\implies f(x_1)\ne f(x_2)][/math] (the contrapositive of the above)
Sometimes an injection is denoted [ilmath]\rightarrowtail[/ilmath][2] (and a surjection [ilmath]\twoheadrightarrow[/ilmath] and a bijection is both of these combined (as if super-imposed on top of each other) - there is no LaTeX arrow for this however) - we do not use this convention.
Statements
Notes
Terminology
- An injective function is sometimes called an embedding[1]
- Just as surjections are called 'onto' an injection may be called 'into'[3] however this is rare and something I frown upon.
- This is French, from "throwing into" referring to the domain, not elements themselves (as any function takes an element into the codomain, it need not be one-to-one)
- I do not like using the word into but do like onto - I say:
- "But [ilmath]f[/ilmath] maps [ilmath]A[/ilmath] onto [ilmath]B[/ilmath] so...."
- "But [ilmath]f[/ilmath] is an injection so...."
- "As [ilmath]f[/ilmath] is a bijection..."
- I see into used rarely to mean injection, and in fact any function [ilmath]f:X\rightarrow Y[/ilmath] being read as [ilmath]f[/ilmath] takes [ilmath]X[/ilmath] into [ilmath]Y[/ilmath] without meaning injection[1][4]
Properties
- The cardinality of the inverse of an element [math]y\in Y[/math] may be no more than 1
- Note this means it may be zero
- In contrast to a bijection where the cardinality is always 1 (and thus we take the singleton set [math]f^{-1}(y)=\{x\}[/math] as the value it contains, writing [ilmath]f^{-1}(y)=x[/ilmath])
- Note this means it may be zero
See also
References
- ↑ 1.0 1.1 1.2 Analysis: Part 1 - Elements - Krzysztof Maurin
- ↑ Notes On Set Theory - Second Edition - Yiannis Moschovakis
- ↑ http://mathforum.org/library/drmath/view/52454.html
- ↑ Real and Abstract Analysis - Edwin Hewitt and Karl Stromberg
| |||||||||||
