Subset & Subspace Homework: Closed Under Vector Addition & Scalar Multiplication

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SUMMARY

The discussion focuses on identifying sets of vectors in R2 that demonstrate specific closure properties under vector addition and scalar multiplication. The first example provided, S = {(x,y) | x + y = 0}, is incorrectly identified as closed under vector addition but is actually a one-dimensional subspace of R2, thus closed under both operations. Participants emphasize the need for distinct examples: one set that is closed under vector addition but not scalar multiplication, and another that is closed under scalar multiplication but not vector addition.

PREREQUISITES
  • Understanding of vector spaces and subspaces in R2
  • Knowledge of vector addition and scalar multiplication operations
  • Familiarity with closure properties in linear algebra
  • Ability to analyze and construct sets of vectors based on given conditions
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  • Research examples of sets in R2 that are closed under vector addition but not scalar multiplication
  • Investigate sets that are closed under scalar multiplication but not vector addition
  • Study the definitions and properties of vector spaces and subspaces
  • Explore linear combinations and their implications in vector space theory
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Students of linear algebra, educators teaching vector space concepts, and anyone seeking to deepen their understanding of closure properties in vector operations.

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Homework Statement




a) Find a set of vectors in R2 that is closed under vector addition but not under scalar multiplication
Find a set of vectors closed under scalar multiplication but not closed under vector addition.

The Attempt at a Solution



a) Let S be a set of vectors in R2.

S = {(x,y) | x + y =0}
x = (1,1) y = (-1,-1)

To show that S set of vectors is closed under vector addition, x + y must remain in S.

x + y = (x1 + y1, x2+y2) = ( 0,0)

Am I right up till here?
 
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negation said:

Homework Statement

a) Find a set of vectors in R2 that is closed under vector addition but not under scalar multiplication
Find a set of vectors closed under scalar multiplication but not closed under vector addition.

The Attempt at a Solution



a) Let S be a set of vectors in R2.

S = {(x,y) | x + y =0}
x = (1,1) y = (-1,-1)

To show that S set of vectors is closed under vector addition, x + y must remain in S.

x + y = (x1 + y1, x2+y2) = ( 0,0)

Am I right up till here?
No. Your set S is the line whose equation is x + y = 0. This is a line through the origin, and as such, this set is a one dimensional subspace of R2. You need to find a different set of vectors.

Look at the examples in your book or notes. There are probably some examples of sets that are closed under one operation, but not the other.

Also note that this is two problems - one for a set that is closed under vector addition but not under multiplication by a scalar; the other is closed under scalar multiplication but not vector addition.
 
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