matrices

matrices - MAT 312/AMS 351 Notes and Exercises on...

This preview shows pages 1–2. Sign up to view the full content.

MAT 312/AMS 351 Notes and Exercises on Permutations and Matrices. We can represent a permutation π S ( n ) by a matrix M π in the following useful way. If π ( i ) = j , then M π has a 1 in column i and row j ; the entries are 0 otherwise. This M π permutes the unit column vectors e 1 , e 2 , . . . , e n , by matrix multiplication, just the way π permutes 1 , 2 , . . . , n . Example. Suppose n = 6 and π = (1542)(36). Following the rule, we get M π = 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 . We can check: π (1) = 5, and M π ( e 1 ) = 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0 0 = 0 0 0 0 1 0 = e 5 , etc. Proposition 1. For σ, π S ( n ), we have M πσ = M π M σ ; i.e. the matrix corresponding to a composition of permutations is the product of the individual matrices. Proof. On the one hand, M πσ ( e i ) = e πσ ( i ) = e π ( σ ( i )) . On the other hand, M π M σ ( e i ) = M π ( e σ ( i ) ) = e π ( σ ( i )) also. square To proceed we need some facts about determinants. (1) Every square matrix M has a determinant det M , which is a sum of products of entries in M . So if M has integer entries, det M will be an integer, etc. (2) The determinant of a 1 × 1 matrix ( a 11 ) is the number a 11 itself.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern