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This module introduces the identity matrix and its properties.

When multiplying numbers, the number 1 has a special property: when you multiply 1 by any number, you get that same number back. We can express this property as an algebraic generalization:

1 x = x

The matrix that has this property is referred to as the identity matrix .

Definition of identity matrix

The identity matrix , designated as [ I ] , is defined by the property: [ A ] [ I ] = [ I ] [ A ] = [ A ]

Note that the definition of [I] stipulates that the multiplication must commute —that is, it must yield the same answer no matter which order you multiply in. This is important because, for most matrices, multiplication does not commute.

What matrix has this property? Your first guess might be a matrix full of 1s, but that doesn’t work:

1 2 3 4 size 12{ left [ matrix { 1 {} # 2 {} ##3 {} # 4{} } right ]} {} 1 1 1 1 size 12{ left [ matrix { 1 {} # 1 {} ##1 {} # 1{} } right ]} {} = 3 3 7 7 size 12{ left [ matrix { 3 {} # 3 {} ##7 {} # 7{} } right ]} {} so 1 1 1 1 size 12{ left [ matrix { 1 {} # 1 {} ##1 {} # 1{} } right ]} {} is not an identity matrix

The matrix that does work is a diagonal stretch of 1s, with all other elements being 0.

1 2 3 4 size 12{ left [ matrix { 1 {} # 2 {} ##3 {} # 4{} } right ]} {} 1 0 0 1 size 12{ left [ matrix { 1 {} # 0 {} ##0 {} # 1{} } right ]} {} = 1 2 3 4 size 12{ left [ matrix { 1 {} # 2 {} ##3 {} # 4{} } right ]} {} so 1 0 0 1 size 12{ left [ matrix { 1 {} # 0 {} ##0 {} # 1{} } right ]} {} is the identity for 2x2 matrices
2 5 9 π 2 8 3 1 / 2 8 . 3 size 12{ left [ matrix { 2 {} # 5 {} # 9 {} ##π {} # - 2 {} # 8 {} ## - 3 {} # 1/2 {} # 8 "." 3{}} right ]} {} 1 0 0 0 1 0 0 0 1 size 12{ left [ matrix { 1 {} # 0 {} # 0 {} ##0 {} # 1 {} # 0 {} ## 0 {} # 0 {} # 1{}} right ]} {} = 2 5 9 π 2 8 3 1 / 2 8 . 3 size 12{ left [ matrix { 2 {} # 5 {} # 9 {} ##π {} # - 2 {} # 8 {} ## - 3 {} # 1/2 {} # 8 "." 3{}} right ]} {} 1 0 0 0 1 0 0 0 1 size 12{ left [ matrix { 1 {} # 0 {} # 0 {} ##0 {} # 1 {} # 0 {} ## 0 {} # 0 {} # 1{}} right ]} {} is the identity for 3x3 matrices

You should confirm those multiplications for yourself, and also confirm that they work in reverse order (as the definition requires).

Hence, we are led from the definition to:

The identity matrix

For any square matrix, its identity matrix is a diagonal stretch of 1s going from the upper-left-hand corner to the lower-right, with all other elements being 0. Non-square matrices do not have an identity. That is, for a non-square matrix [ A ] , there is no matrix such that [ A ] [ I ] = [ I ] [ A ] = [ A ] .

Why no identity for a non-square matrix? Because of the requirement of commutativity. For a non-square matrix [ A ] you might be able to find a matrix [ I ] such that [ A ] [ I ] = [ A ] ; however, if you reverse the order, you will be left with an illegal multiplication.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Advanced algebra ii: conceptual explanations. OpenStax CNX. May 04, 2010 Download for free at http://cnx.org/content/col10624/1.15
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