3D spaces in $mathbb{R}^4$












0












$begingroup$


Let $V$ be a 3-dimensional subspace in $mathbb{R}^4$. Find such $V_1, V_2, V_3, V_4$ that $$V_1 cap V_2 cap V_3 cap V_4 = {0,0,0,0}$$.



I thought about such $V_i$ that
$V_1 = left{ begin{bmatrix}
1\
0\
0\
0\
end{bmatrix},
begin{bmatrix}
0\
1\
0\
0\
end{bmatrix},
begin{bmatrix}
0\
0\
1\
0\
end{bmatrix} right}
$



And $V_2$ is the same, but the bottom line changes to 1, in $V_3$ it changes to 2 and in $V_4$ to 3. Am I thinking right?










share|cite|improve this question









$endgroup$

















    0












    $begingroup$


    Let $V$ be a 3-dimensional subspace in $mathbb{R}^4$. Find such $V_1, V_2, V_3, V_4$ that $$V_1 cap V_2 cap V_3 cap V_4 = {0,0,0,0}$$.



    I thought about such $V_i$ that
    $V_1 = left{ begin{bmatrix}
    1\
    0\
    0\
    0\
    end{bmatrix},
    begin{bmatrix}
    0\
    1\
    0\
    0\
    end{bmatrix},
    begin{bmatrix}
    0\
    0\
    1\
    0\
    end{bmatrix} right}
    $



    And $V_2$ is the same, but the bottom line changes to 1, in $V_3$ it changes to 2 and in $V_4$ to 3. Am I thinking right?










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      Let $V$ be a 3-dimensional subspace in $mathbb{R}^4$. Find such $V_1, V_2, V_3, V_4$ that $$V_1 cap V_2 cap V_3 cap V_4 = {0,0,0,0}$$.



      I thought about such $V_i$ that
      $V_1 = left{ begin{bmatrix}
      1\
      0\
      0\
      0\
      end{bmatrix},
      begin{bmatrix}
      0\
      1\
      0\
      0\
      end{bmatrix},
      begin{bmatrix}
      0\
      0\
      1\
      0\
      end{bmatrix} right}
      $



      And $V_2$ is the same, but the bottom line changes to 1, in $V_3$ it changes to 2 and in $V_4$ to 3. Am I thinking right?










      share|cite|improve this question









      $endgroup$




      Let $V$ be a 3-dimensional subspace in $mathbb{R}^4$. Find such $V_1, V_2, V_3, V_4$ that $$V_1 cap V_2 cap V_3 cap V_4 = {0,0,0,0}$$.



      I thought about such $V_i$ that
      $V_1 = left{ begin{bmatrix}
      1\
      0\
      0\
      0\
      end{bmatrix},
      begin{bmatrix}
      0\
      1\
      0\
      0\
      end{bmatrix},
      begin{bmatrix}
      0\
      0\
      1\
      0\
      end{bmatrix} right}
      $



      And $V_2$ is the same, but the bottom line changes to 1, in $V_3$ it changes to 2 and in $V_4$ to 3. Am I thinking right?







      linear-algebra






      share|cite|improve this question













      share|cite|improve this question











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      share|cite|improve this question










      asked Feb 1 at 22:23









      aaaaaa

      133




      133






















          2 Answers
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          1












          $begingroup$

          You are correct assuming I'm interpreting what you've said in the right way. Essentially you're letting $e_i$ be the $i$-th standard unit basis vector, i.e.
          $$e_1 = begin{pmatrix}1&0&0&0 end{pmatrix}, e_2 = begin{pmatrix}0&1&0&0 end{pmatrix} ,..., e_4 = begin{pmatrix} 0&0&0&1end{pmatrix}$$

          and then letting
          $$V_1 = text{span} {e_1, e_2, e_3 }, V_2 = text{span}{e_2,e_3,e_4}, V_3 = text{span}{e_1,e_3,e_4}, V_4= {e_1,e_2,e_4} $$
          As such, for $v_1$ to be in $V_1$, its fourth coordinate must be $0$. Further, for $v_1$ to be in $V_1 cap V_2$, both it's fourth and first coordinate must be zero. This goes on until we see that for $v_1$ to be in $cap_{i=1}^4 V_i$ that all coordinates of $v_1$ must be $0$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
            $endgroup$
            – aaa
            Feb 1 at 22:46








          • 1




            $begingroup$
            @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
            $endgroup$
            – Daniel Schepler
            Feb 1 at 23:10





















          0












          $begingroup$

          You're working with vector spaces?



          If so, then the idea will work. Take a nice simple basis with four elements: ${e_1, e_2, e_3, e_4}$. Let $V_n$ be the subspace generated by this basis with $e_n$ removed.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
            $endgroup$
            – aaa
            Feb 1 at 22:52












          • $begingroup$
            I got your idea. I just thought that my slight variation was a little neater.
            $endgroup$
            – badjohn
            Feb 1 at 22:56










          • $begingroup$
            Yes, your is very elegant. Thank you.
            $endgroup$
            – aaa
            Feb 1 at 22:57














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          2 Answers
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          active

          oldest

          votes








          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1












          $begingroup$

          You are correct assuming I'm interpreting what you've said in the right way. Essentially you're letting $e_i$ be the $i$-th standard unit basis vector, i.e.
          $$e_1 = begin{pmatrix}1&0&0&0 end{pmatrix}, e_2 = begin{pmatrix}0&1&0&0 end{pmatrix} ,..., e_4 = begin{pmatrix} 0&0&0&1end{pmatrix}$$

          and then letting
          $$V_1 = text{span} {e_1, e_2, e_3 }, V_2 = text{span}{e_2,e_3,e_4}, V_3 = text{span}{e_1,e_3,e_4}, V_4= {e_1,e_2,e_4} $$
          As such, for $v_1$ to be in $V_1$, its fourth coordinate must be $0$. Further, for $v_1$ to be in $V_1 cap V_2$, both it's fourth and first coordinate must be zero. This goes on until we see that for $v_1$ to be in $cap_{i=1}^4 V_i$ that all coordinates of $v_1$ must be $0$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
            $endgroup$
            – aaa
            Feb 1 at 22:46








          • 1




            $begingroup$
            @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
            $endgroup$
            – Daniel Schepler
            Feb 1 at 23:10


















          1












          $begingroup$

          You are correct assuming I'm interpreting what you've said in the right way. Essentially you're letting $e_i$ be the $i$-th standard unit basis vector, i.e.
          $$e_1 = begin{pmatrix}1&0&0&0 end{pmatrix}, e_2 = begin{pmatrix}0&1&0&0 end{pmatrix} ,..., e_4 = begin{pmatrix} 0&0&0&1end{pmatrix}$$

          and then letting
          $$V_1 = text{span} {e_1, e_2, e_3 }, V_2 = text{span}{e_2,e_3,e_4}, V_3 = text{span}{e_1,e_3,e_4}, V_4= {e_1,e_2,e_4} $$
          As such, for $v_1$ to be in $V_1$, its fourth coordinate must be $0$. Further, for $v_1$ to be in $V_1 cap V_2$, both it's fourth and first coordinate must be zero. This goes on until we see that for $v_1$ to be in $cap_{i=1}^4 V_i$ that all coordinates of $v_1$ must be $0$.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
            $endgroup$
            – aaa
            Feb 1 at 22:46








          • 1




            $begingroup$
            @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
            $endgroup$
            – Daniel Schepler
            Feb 1 at 23:10
















          1












          1








          1





          $begingroup$

          You are correct assuming I'm interpreting what you've said in the right way. Essentially you're letting $e_i$ be the $i$-th standard unit basis vector, i.e.
          $$e_1 = begin{pmatrix}1&0&0&0 end{pmatrix}, e_2 = begin{pmatrix}0&1&0&0 end{pmatrix} ,..., e_4 = begin{pmatrix} 0&0&0&1end{pmatrix}$$

          and then letting
          $$V_1 = text{span} {e_1, e_2, e_3 }, V_2 = text{span}{e_2,e_3,e_4}, V_3 = text{span}{e_1,e_3,e_4}, V_4= {e_1,e_2,e_4} $$
          As such, for $v_1$ to be in $V_1$, its fourth coordinate must be $0$. Further, for $v_1$ to be in $V_1 cap V_2$, both it's fourth and first coordinate must be zero. This goes on until we see that for $v_1$ to be in $cap_{i=1}^4 V_i$ that all coordinates of $v_1$ must be $0$.






          share|cite|improve this answer









          $endgroup$



          You are correct assuming I'm interpreting what you've said in the right way. Essentially you're letting $e_i$ be the $i$-th standard unit basis vector, i.e.
          $$e_1 = begin{pmatrix}1&0&0&0 end{pmatrix}, e_2 = begin{pmatrix}0&1&0&0 end{pmatrix} ,..., e_4 = begin{pmatrix} 0&0&0&1end{pmatrix}$$

          and then letting
          $$V_1 = text{span} {e_1, e_2, e_3 }, V_2 = text{span}{e_2,e_3,e_4}, V_3 = text{span}{e_1,e_3,e_4}, V_4= {e_1,e_2,e_4} $$
          As such, for $v_1$ to be in $V_1$, its fourth coordinate must be $0$. Further, for $v_1$ to be in $V_1 cap V_2$, both it's fourth and first coordinate must be zero. This goes on until we see that for $v_1$ to be in $cap_{i=1}^4 V_i$ that all coordinates of $v_1$ must be $0$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 1 at 22:35









          Theo C.Theo C.

          44228




          44228












          • $begingroup$
            Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
            $endgroup$
            – aaa
            Feb 1 at 22:46








          • 1




            $begingroup$
            @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
            $endgroup$
            – Daniel Schepler
            Feb 1 at 23:10




















          • $begingroup$
            Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
            $endgroup$
            – aaa
            Feb 1 at 22:46








          • 1




            $begingroup$
            @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
            $endgroup$
            – Daniel Schepler
            Feb 1 at 23:10


















          $begingroup$
          Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
          $endgroup$
          – aaa
          Feb 1 at 22:46






          $begingroup$
          Thank you very much! Yes, what I've said doesn't seem very clear as I read it now. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}.
          $endgroup$
          – aaa
          Feb 1 at 22:46






          1




          1




          $begingroup$
          @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
          $endgroup$
          – Daniel Schepler
          Feb 1 at 23:10






          $begingroup$
          @aaa That example you gave doesn't work: for example, $V_3 = { x in mathbb{R}^4 mid x_4 = 2(x_1 + x_2 + x_3) }$, and then by combining similar descriptions for the others, $(1, 1, -2, 0)$ is in the intersection. Overall, the intersection would be ${ x in mathbb{R}^4 mid x_1 + x_2 + x_3 = x_4 = 0 }$.
          $endgroup$
          – Daniel Schepler
          Feb 1 at 23:10













          0












          $begingroup$

          You're working with vector spaces?



          If so, then the idea will work. Take a nice simple basis with four elements: ${e_1, e_2, e_3, e_4}$. Let $V_n$ be the subspace generated by this basis with $e_n$ removed.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
            $endgroup$
            – aaa
            Feb 1 at 22:52












          • $begingroup$
            I got your idea. I just thought that my slight variation was a little neater.
            $endgroup$
            – badjohn
            Feb 1 at 22:56










          • $begingroup$
            Yes, your is very elegant. Thank you.
            $endgroup$
            – aaa
            Feb 1 at 22:57


















          0












          $begingroup$

          You're working with vector spaces?



          If so, then the idea will work. Take a nice simple basis with four elements: ${e_1, e_2, e_3, e_4}$. Let $V_n$ be the subspace generated by this basis with $e_n$ removed.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
            $endgroup$
            – aaa
            Feb 1 at 22:52












          • $begingroup$
            I got your idea. I just thought that my slight variation was a little neater.
            $endgroup$
            – badjohn
            Feb 1 at 22:56










          • $begingroup$
            Yes, your is very elegant. Thank you.
            $endgroup$
            – aaa
            Feb 1 at 22:57
















          0












          0








          0





          $begingroup$

          You're working with vector spaces?



          If so, then the idea will work. Take a nice simple basis with four elements: ${e_1, e_2, e_3, e_4}$. Let $V_n$ be the subspace generated by this basis with $e_n$ removed.






          share|cite|improve this answer









          $endgroup$



          You're working with vector spaces?



          If so, then the idea will work. Take a nice simple basis with four elements: ${e_1, e_2, e_3, e_4}$. Let $V_n$ be the subspace generated by this basis with $e_n$ removed.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Feb 1 at 22:38









          badjohnbadjohn

          4,4171720




          4,4171720












          • $begingroup$
            Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
            $endgroup$
            – aaa
            Feb 1 at 22:52












          • $begingroup$
            I got your idea. I just thought that my slight variation was a little neater.
            $endgroup$
            – badjohn
            Feb 1 at 22:56










          • $begingroup$
            Yes, your is very elegant. Thank you.
            $endgroup$
            – aaa
            Feb 1 at 22:57




















          • $begingroup$
            Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
            $endgroup$
            – aaa
            Feb 1 at 22:52












          • $begingroup$
            I got your idea. I just thought that my slight variation was a little neater.
            $endgroup$
            – badjohn
            Feb 1 at 22:56










          • $begingroup$
            Yes, your is very elegant. Thank you.
            $endgroup$
            – aaa
            Feb 1 at 22:57


















          $begingroup$
          Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
          $endgroup$
          – aaa
          Feb 1 at 22:52






          $begingroup$
          Hello. Yes, these are vector spaces. Actually I meant something like that: V1 = {(1,0,0,0), (0,1,0,0), (0,0,1,0)}, V2 = {(1,0,0,1), (0,1,0,1), (0,0,1,1)}, V3 = {(1,0,0,2), (0,1,0,2), (0,0,1,2)}, V4 = {(1,0,0,3), (0,1,0,3), (0,0,1,3)}. And thank you for your answer!
          $endgroup$
          – aaa
          Feb 1 at 22:52














          $begingroup$
          I got your idea. I just thought that my slight variation was a little neater.
          $endgroup$
          – badjohn
          Feb 1 at 22:56




          $begingroup$
          I got your idea. I just thought that my slight variation was a little neater.
          $endgroup$
          – badjohn
          Feb 1 at 22:56












          $begingroup$
          Yes, your is very elegant. Thank you.
          $endgroup$
          – aaa
          Feb 1 at 22:57






          $begingroup$
          Yes, your is very elegant. Thank you.
          $endgroup$
          – aaa
          Feb 1 at 22:57




















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