# solutions2.1 - Math 307 Problems for section 2.1 0 0 1 1 1...

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Math 307: Problems for section 2.1 October 22, 2015 1. Are the vectors , - 2 1 1 , - 2 0 , - 2 0 1 , - 9 7 3 LAB/Octave to perform calculations, but explain your answer. 1 2 1 2 1 1 0 1 - 1 3 0 0 0 4 linearly independent? You may use MAT- rref(A)
ans = 1.00000 0.00000 0.00000 0.00000 1.00000 0.00000 1.00000 0.00000 0.00000 1.00000 0.00000 0.00000 1.00000 0.00000 -2.00000 0.00000 0.00000 0.00000 1.00000 1.00000 0.00000 0.00000 0.00000 0.00000 0.00000 Since the matrix has rank 4 there are only 4 independent vectors in the list, and the vectors are linearly dependent. In fact a vector in the nullspace of A is - 1 - 1 2 - 1 1 so - V 1 - V 2 + 2 V 3 - V 4 + V 5 = 0 which we can check: -V1 - V2 + 2*V3 - V4 + V5 ans = 0 0 0 0 0 1
2. Which of the following sets are subspaces of the vector space V ? Why, or why not? R )
(e) The set of all polynomial functions, p ( x ) , where p ( x ) = 0 or p ( x ) has degree n for some fixed n 1 . ( V is the vector space of all polynomials.) (f) The set of odd continuous functions on the interval [ - 1 , 1] , i.e., f C [ - 1 , 1] such that f ( - x ) = - f ( x ) . ( V = C [ - 1 , 1] ) (a) S is a subspace. The zero vector, (0 , 0 , 0), is in S . S is also closed under addition and scalar multiplication. Let x 1 = (0 , y 1 , z 1 ) and x 2 = (0 , y 2 , z 2 ) be two arbitrary vectors in S and let c R . Then x 1 + x 2 = (0 , y 1 + y 2 , z 1 + z 2 ) is also in S and c x 1 = (0 , cy 1 , cz 1 ) is also in S . (b) S is not a subspace. It is not closed under addition. Take for example the vectors (0 , 1 , 1) and (1 , 0 , 1). They are both in S but their sum, (1 , 1 , 2), is not in S . (c) This is a subspace. The zero vector clearly has x j = 0 from some point onwards. If we add two vectors which both have x j = 0 from some point onwards then the sum will also have x j = 0 from the same point onwards. Also, if we multiply a vector with x j = 0 from some point onwards by a scalar, the result will still have x j = 0 from the same point on. So the set is closed under addition and scalar multiplication. (d) This is not a subspace. It is not closed under scalar multiplication. Take for example the vector (2 , 1 , 0 , 0 , 0 , . . . ). If we multiply this by ( - 1) we get ( - 2 , - 1 , 0 , 0 , 0 , . . . ), which is no longer non- increasing. (e) This is not a subspace. Take for example the two polynomials p 1 ( x ) = x n + 1 and p 2 ( x ) = - x n . These are both of degree n , but their sum p 1 ( x ) + p 2 ( x ) = 1 is not a polynomial of degree n . So the set is not closed under addition.
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