# Jordan Canonical Form. Jordan Normal Form.

Just had a couple of quick questions regarding Jordan Normal Form (JNF).

1. Can all matrices be put into JNF?

2. What is the difference between JNF and Jordan Canonical Form.

3. What are JNF and JCF useful for?

Thank you very much :)

Yes, all matrices can be put in Jordan Normal Form.

There is no difference between "Jordan Normal Form" and "Jordan Canonical Form". They are different names for the same thing. (I had never heard of "Jordan Canonical Form" but a search quickly gave https://www.google.com/search?q=jordan+canonical+form&ie=&oe= ) The "Jordan Normal Form" (= "Jordan Canonical Form") Allows us to write a matrix in a simplified form. $A= B^{-1}JB$ for some invertible matrix B. Writing it as a diagonal matrix would be even simpler but not every matrix is "invertible". (If a matrix is invertible then it "Jordan Normal Form" is diagonal.)

• Where you say "invertible" you mean "diagonalisable", I think. Also, the "yes" might need some add on. you could say, "yes, over the complex numbers/" – quid May 11 '17 at 11:39
• Thank you so much for the help. It is much appreciated. Would you suggest any websites or videos that provide a step-by-step method to finding the JNF for any n x n square matrices. Thank you again. – Maths May 11 '17 at 12:16
1. Can all matrices be put into JNF?

The characteristic polynomial of a matrix in JNF factors into linear factors (as it does for any triangular matrix). Conversely, (up to multiplication by a constant) every polynomial is the characteristic polynomial of some matrix.

It thus follows that you need to be willing to work over a field in which every polynomial factors into linear factors, that is over an algebraically closed field.

Concretely, if you insist on working over the reals, the answer is "no." If you allow passage to the complex numbers the answer is "yes."

1. What is the difference between JNF and Jordan Canonical Form.

None. These are two names for the same notion.

1. What are JNF and JCF useful for?

For example:

• They allow a classification of endomorphism/matrices up to base change.
• They can be used to efficiently calculate high powers of a matrix.
• In relation to the above point, they are useful to determine things like the exponential of a matrix, which is in turn useful for solving differential equations.
• Thank you so much for the help. It is much appreciated. Would you suggest any websites or videos that provide a step-by-step method to finding the JNF for any n x n square matrices. Thank you again. – Maths May 11 '17 at 12:16
• You are welcome. The notes on that site seem good to me math.ucla.edu/~jlindquist – quid May 11 '17 at 13:02