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Unformatted text preview: 126 2. BASIC THEORY OF GROUPS not a linear space. So first explore the condition for a matrix A to commute with all matrices B , not just invertible matrices. Note that the condition AB D BA is linear in B , so a matrix A commutes with all matrices if, and only if, it commutes with each member of a linear spanning set of ma trices. So now consider the socalled matrix units E ij , which have a 1 in the i;j position and zeros elsewhere. The set of matrix units is a basis of the linear space of matrices. Find the condition for a matrix to commute with all of the E ij s. It remains to show that if a matrix commutes with all invertible matrices, then it also commutes with all E ij s. (The results of this exercise hold just as well with the real numbers R replaced by the complex numbers C or the rational numbers Q .) 2.5.16. Show that the symmetric group S n has a unique subgroup of index 2, namely the subgroup A n of even permutations. Hint: Such subgroup N is normal. Hence if it contains one element of a certain cycle structure,is normal....
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 Fall '08
 EVERAGE
 Algebra, Matrices

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