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## liliy 3 years ago Show that if A is nxn and has all 0’s on and below the diagonal then An = 0. Hint: do not at first be too ambitious. First find A2 and observe something useful about it. What about A3?

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1. myininaya

What is An?

2. liliy

the matrix

3. liliy

commonly seen as Ax=b. this is An=0.

4. myininaya

Oh I thought there might have been a difference because you called the matrix A then you called it An

5. myininaya

Are you saying n is an eigenvector ?

6. liliy

honesly i dont know wat to do. this is what the teacher asked us

7. myininaya

Or are you say An is the matrix?

8. liliy

idk..lol

9. myininaya

Ok I think |dw:1340595284758:dw| and |dw:1340595304816:dw|

10. myininaya

Is that what you think?

11. myininaya

But that doesn't make since that A_n would be the matrix with nothing but zero entries

12. myininaya

do you mean the determinant is 0?

13. myininaya

|A_n|=0?

14. myininaya

that would make since

15. myininaya

sense*

16. myininaya

I think that is what you mean

17. myininaya

so do you know how to find the determinant of a matrix?

18. liliy

ya ad-bc

19. myininaya

Try finding the determinant of A_2 ? What do you get?

20. liliy

zero

21. myininaya

Ok what about A_3

22. liliy

same

23. myininaya

Ok so we have convinced ourselves that |A_n|=0 But we must prove it

24. myininaya

|dw:1340595692432:dw|

25. myininaya

I would just show a little work for this show a pattern you know

26. myininaya

like how you did for A_3 and then do the nth term you know what I mean?

27. liliy

how do u find determinant for 3x3 or bigger matrix?

28. myininaya

oh ok for an A_3 |dw:1340595913293:dw|

29. myininaya

|dw:1340595950507:dw|

30. myininaya

The signs alternate

31. myininaya

Like you take top entries

32. liliy

only the first row?

33. myininaya

And take everything that isn't below that entry

34. liliy

like if its 10x10 u still only do the first row /?

35. myininaya

For |A_4| |dw:1340596049002:dw|

36. myininaya

|dw:1340596102188:dw|

37. myininaya

and you already know how to find the determinant for a 3 by 3

38. myininaya

Same thing just take the top entries and do the signs alternating

39. eliassaab

$A=\left( \begin{array}{cccc} 0 & 1 & 2 & 3 \\ 0 & 0 & 1 & 4 \\ 0 & 0 & 0 & 3 \\ 0 & 0 & 0 & 0 \\ \end{array} \right)\\ A^2=\left( \begin{array}{cccc} 0 & 0 & 1 & 10 \\ 0 & 0 & 0 & 3 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ \end{array} \right)\\ A^3=\left( \begin{array}{cccc} 0 & 0 & 0 & 3 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ \end{array} \right)\\ A^4=\left( \begin{array}{cccc} 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ \end{array} \right)$

40. liliy

but the determinant of the new 3x3 is gonna also be broken down right? .. im ur case its zero bec the coefficient is zero so it odsnt really mater

41. myininaya

yes just like i did above for the 3 by 3

42. myininaya

But not all the top entries are 0

43. liliy

@eliassaab i dont really understnad wat u wrote

44. liliy

right...

45. eliassaab

See my example below and examine what is going on?

46. liliy

you have zeros. but what are you doing to the matrix?

47. myininaya

Do you think he means to raise A to a power @eliassaab ?

48. myininaya

Instead of finding the determinant ?

49. eliassaab

You raise it to the power 2, then 3, then 4.

50. myininaya

Ok I'm sorry @liliy I don't know what your question is asking anymore.

51. eliassaab

You do not need to deal with determina

52. eliassaab

determinant

53. liliy

so can you start over with me?

54. liliy

what does a^n=0 even mean?

55. eliassaab

Any matrix like yours, when you raise it to the power 2, you get what is first above the diagonal is zero When you raise it to the power 2, you get the first and the second above the diagonal to be zero. When you raise it to the power 3, you get the first and the second and third above the diagonal to be zero. When you raise it to the power 4, you get everything zero.

56. eliassaab

Look at A^4 in my example above to see that A^4=0, this means all the entries of the matrix A^4 are zeros.

57. myininaya

So you are just giving another way right @eliassaab Do you think I interpreted is question correctly?

58. eliassaab

@myininaya, you do not need determinant to do that,

59. myininaya

Yeah I know, but I'm asking you if I interpreted it correctly?

60. liliy

@eliassaab i dont undesrtnad how you started to do the problem. my teacher said start with a^2 .. and move to bigger ones... so wat is a= to a 4x4 and then writing a^2... a^3..

61. eliassaab

Here is a quick proof using the characteristic polynomial f(x) of the matrix A that says the f(A)=0. Our matrix has$$f(x)=x^n$$, hence $$f(A)=A^n=0$$

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