Question #150979
Let V be the vector space of all n x n
matrices over R. What is the dimension of
V over R ? Further, let
Wn={Anxn belongs to V I Anxn is upper triangular}.
Check whether or not W is a subspace of V.
1
Expert's answer
2020-12-15T03:49:51-0500

Let VV be the vector space of all n×nn \times n matrices over R\mathbb R.  For i,j{1,2,...,n}i,j\in\{1,2,...,n\} denote by EijE_{ij} the matrix with a unigue 1 at the intersection of the row ii and the column jj, and with the rest elements equal to 0: eij=1e_{ij}=1 and est=0e_{st}=0 for all sis\ne i and tjt\ne j. Then (Eij  i,j{1,...,n})(E_{ij}\ |\ i,j\in\{1,..., n\}) is a basis of the vector space VV. Indeed, if i=1nj=1naijEij=O\sum\limits_{i=1}^n\sum\limits_{j=1}^na_{ij}E_{ij}=O where the matrix OO contains of only zeros, then aij=0a_{ij}=0 for all i,j{1,2,...,n}i,j\in\{1,2,...,n\}. Consequently, the matrices EijE_{ij} are linear independent. Also each matrix MM with entries mijm_{ij} can be represented as M=i=1nj=1nmijEijM=\sum\limits_{i=1}^n\sum\limits_{j=1}^nm_{ij}E_{ij}, and therefore, (Eij  i,j{1,...,n})(E_{ij}\ |\ i,j\in\{1,..., n\}) indeed a basis of VV. It follows that dim(V)=n2.\dim(V)=n^2.



Further, let Wn={An×nV  An×n is upper triangular}.W_n=\{A_{n\times n} \in V \ |\ A_{n\times n}\text{ is upper triangular}\}. Let  An×n,Bn×n{\displaystyle A_{n\times n}, B_{n\times n}} be elements of WnW_n  and α,β{\displaystyle \alpha ,\beta }  be elements of R\mathbb R. Since the elements under the main diagonal in matrices  An×n,Bn×n{\displaystyle A_{n\times n}, B_{n\times n}} are zeros, it follows that the elements under the main diagonal in the matrix αAn×n+βBn×n{\displaystyle\alpha A_{n\times n}+\beta B_{n\times n}} is zeros as well. Therefore,  αAn×n+βBn×n{\displaystyle\alpha A_{n\times n}+\beta B_{n\times n}}  is in WnW_n, and we conclude that WnW_n is a subspace of VV.



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