## MathSofiya 2 years ago I'm trying to find a pattern $a_{n+2}=\frac{a_n}{(n+2)(n+1)}$ $n=0:a_2=\frac{a_0}{2!}$ $n=2:a_4=\frac{a_0}{4!}$ but that pattern changes because the next term is not $\frac{a_0}{6!}$ $n=4:a_6=\frac{a_0}{2\cdot 30}=$

1. Algebraic!

?? $a _{4} = \frac{a _{0} }{ 4! }$

2. TuringTest

this is the one we did yesterday? wasn't it$a_{n+2}=-a_n\frac{1-n}{(n+1)(n+2)}$?

3. MathSofiya

no this is new problem $y''=y$

4. TuringTest

oooooooh

5. MathSofiya

I'll write out what I have so far

6. TuringTest

I'll be able to able to help in about 15-20 min

7. MathSofiya

sure

8. MathSofiya

$\sum_{n=2}^\infty n(n-1)a_nx^{n-2}-\sum_{n=0}^\infty a_nx^n=0$

9. nipunmalhotra93

@MathSofiya how exactly is the pattern changing?

10. TuringTest

let's establish that we have the right pattern first... I feel like you may have missed something continue plz @MathSofiya so far so good

11. MathSofiya

$\sum_{n=0}^\infty (n+2)(n+1)a_{n+2}x^n-\sum_{n=0}^\infty a_nx^n=0$

12. TuringTest

so yeah, right pattern

13. MathSofiya

$\sum_{n=0}^\infty x^n\left[(n+2)(n+1)-a_n\right]=0$

14. TuringTest

$\sum_{n=0}^\infty x^n\left[(n+2)(n+1)a_{n+2}-a_n\right]=0$

15. MathSofiya

oops

16. TuringTest

$a_{n+2}=\frac{a_n}{(n+1)(n+2)}$so you were right, just plug in each n and write out $$everything$$ explicitly, even multiplication by 1

17. MathSofiya

ok

18. MathSofiya

$n=0:a_{0+2}=a_2=\frac {a_0}{2}$ $n=1:a_{1+2}=a_3=\frac {a_1}{6}$ $n=2:a_{2+2}=a_4=\frac {a_2}{12}$ $n=3:a_{3+2}=a_5=\frac {a_3}{20}$ $n=4:a_{4+2}=a_6=\frac {a_4}{30}$ $n=5:a_{5+2}=a_7=\frac {a_5}{42}$

19. MathSofiya

so we established the relationship between $a_2=\frac{a_0}{2}$ but how do I continue this to the next line? how do I change a_2 to a_0?

20. MathSofiya

ooohh no need to because the next line is going to be zero

21. TuringTest

you are not writing it out explicitly enough to see the pattern reduce everything to terms of either a_0 or a_1 the two constants we can represent no more simply

22. TuringTest

now is the next line zero?

23. TuringTest

how*

24. MathSofiya

well it was yesterday LOL ....Ok I'll write everything out

25. TuringTest

$n=0:a_{0+2}=a_2=\frac {a_0}{2\cdot1}$$n=1:a_3=\frac{a_1}{3\cdot2}$write everything this way, with each number on the bottom specified

26. TuringTest

yesterday we had n-1 in the top, that's why

27. TuringTest

Haha I'm exercising while you crunch the numbers :P

28. MathSofiya

$n=0:a_{0+2}=a_2=\frac {a_0}{2}$ $n=1:a_{1+2}=a_3=\frac {a_1}{6}=\frac{a_1}{2\cdot 3}$ $n=2:a_{2+2}=a_4=\frac {a_2}{12}=\frac{a_0}{2\cdot2\cdot3}$ $n=3:a_{3+2}=a_5=\frac {a_3}{20}=\frac{a_3}{10\cdot2}=\frac{a_1}{2\cdot2\cdot2\cdot3\cdot5}$ $n=4:a_{4+2}=a_6=\frac {a_4}{30}=\frac{a_0}{2\cdot2\cdot2\cdot3\cdot5}$ $n=5:a_{5+2}=a_7=\frac {a_5}{42}=\frac{a_1}{2\cdot2\cdot2\cdot3\cdot5\cdot42}$

29. MathSofiya

n=4 is wrong oh well...but I see a pattern now. How do you workout and do math?

30. TuringTest

push ups, then run over and check your work real quick :)

31. TuringTest

n=2 is wrong

32. TuringTest

12 is not 2x3

33. MathSofiya

haha! That's what I call commitment!

34. MathSofiya

give me a second to do the problem on paper. It's kinda tedious to type it...one second

35. TuringTest

$a_n=\frac{a_n}{(n+1)(n+2)}$$n=0:a_{0+2}=a_2=\frac {a_0}{2\cdot1}$$n=1:a_3=\frac{a_1}{3\cdot2}$$n=2:a_4=\frac {a_2}{4\cdot3}=\frac{a_0}{4\cdot3\cdot2\cdot1}$$n=3:a_5=\frac {a_3}{5\cdot4}=\frac{a_1}{5\cdot4\cdot3\cdot2}$

36. TuringTest

write out each number in the denom without simplifying every time

37. TuringTest

I cannot stress that enough to find patterns

38. MathSofiya

$n=0:a_{0+2}=a_2=\frac {a_0}{2}$ $n=1:a_{1+2}=a_3=\frac {a_1}{6}=\frac{a_1}{3\cdot 2}$ $n=2:a_{2+2}=a_4=\frac {a_2}{12}=\frac{a_0}{4\cdot3\cdot2\cdot1}$ $n=3:a_{3+2}=a_5=\frac {a_3}{20}=\frac{a_3}{10\cdot2}=\frac{a_1}{5\cdot4\cdot3\cdot2\cdot1}$ Ok I'm finally convinced...had to calculate like 5 times that the denominator is actually 120

39. TuringTest

good, but more importantly what is/are the general pattern(s)?

40. TuringTest

again, there are two; one for even n and one for odd n

41. MathSofiya

sorry internet is super slow. I'm gonna restart my computer brb

42. MathSofiya

ok I'm back

43. TuringTest

I'm getting in the shower soon... so what are the two constants that every other constant $$a_n$$ can be written in terms of ?

44. MathSofiya

I wanna say $(n+2)!$ and then sub in n=2k

45. MathSofiya

wait...you go shower, and let me work on this

46. TuringTest

no, and that would only be evens, and needs a base constant like $$a_0$$ write it as$a_n=?$and remember this time we do have odd terms

47. MathSofiya

allow me to decipher it...

48. MathSofiya

The denominator for both even and odds is the following...still working on the numerator $\frac{?}{(n+2)!}$

49. TuringTest

look at the difference between constants with an even and odd subscript what is the main difference you see>?

50. MathSofiya

one will always be zero and the other will always be 1. I have come to that conclusion, but now I have to find a way to write it mathematically...I'm gonna cheat and look at yesterdays work

51. TuringTest

no wait, you can do it... let me give another hint...

52. MathSofiya

ok

53. TuringTest

odd are reducible to being in terms of the constant a_1 evens are reducible to being in terms of a_0 and for the denominator for any constant $$a_n$$ what is it? look at the denom for a_2, a_3, a_4, what are they in terms of n ?

54. TuringTest

don't get confused with the n=(whatever) thing I mean what is the relation between the n in the subscript $$a_n$$ and the number in the denominator?

55. MathSofiya

hold on I didn't write anything yet

56. MathSofiya

57. TuringTest

no rush, still haven't showered yet

58. MathSofiya

When the numerator is $a_0$ the denominator is even When the numerator is $a_1$ the denominator is odd

59. MathSofiya

by odd I mean 5! 3! and so on

60. TuringTest

exactly, and all even numbers can be written how?

61. MathSofiya

2k

62. TuringTest

and odd?

63. MathSofiya

n=2k

64. MathSofiya

n=2k+1

65. MathSofiya

$a_{2k}$ and $a_{2k+1}$

66. TuringTest

so then what is our formulas? we need one for all $$a_{2k}$$ and another for all $$a_2k+1}$$

67. TuringTest

right, we need a formula for each one.. what are they ? that's probably the hardest part of the problem in my opinion is finding the pattern

68. MathSofiya

I would like to put a_{2k} in the numerator and (n+2) in the numerator, but that wouldn't be right. So I have write it in one formula to account for both the even and odd?

69. MathSofiya

I can't write two separate formulas correct? I've gotta write one?

70. TuringTest

everything should be written in terms of either a_0 and k, or a_1 and k n is now out of the poicture completely

71. TuringTest

picture*

72. MathSofiya

gtg...I know this is the last part, and shouldn't take too long, but I gtg to the gym. i'll be back at 7:30 8pm

73. MathSofiya

ok bye

74. TuringTest

cool, health makes one think more clearly I believe see ya

75. MathSofiya

LOL I hope soo! see ya

76. MathSofiya

where is Alan Turing?

77. MathSofiya

I'm reviewing the previous problems we did...and there might be some errors. I not convinced with the way that the final answer was written in k's

78. TuringTest

Allow me to restore your faith, though there are other manners of making the notation I'm sure... Paul to the rescue again: http://tutorial.math.lamar.edu/Classes/DE/SeriesSolutions.aspx Notice that since the constants become different depending on even-odd subscripts for many answers we divide the subscripts in the recursion formula for them into terms of 2k and 2k+1. This particular problem you are doing is almost identical to example 1 one this page, with the exception of the minus between y'' and y exchanged for a plus (reading this example you will probably discover the pattern I was asking you to look for). Notice that it is in fact essential that we change the terms from n to k, since we can get no single summation formula in terms of n since the coefficients depend on whether n is odd n. This is not true for all problems with series solutions, but when we need to distinguish between even and odd n the only way to do this is to put them as two separate series in terms of k. Read example 1 carefully to understand the intricacies of the process, and the why we put n in terms of k. Sorry I never returned last night, I was teaching political philosophy. Look forward to finishing the problem with you today if possible. Tag me when you are ready to continue :)

79. MathSofiya

I'll be back in like 10 mins, gotta start laundry

80. TuringTest

right-o

81. MathSofiya

I can't find my laundry card! Ok so I was trying to rewrite the first problem, and got to the part where 1. all even n#'s =0, or all odd "a" subscripts=0 2. We've gotten a pattern that we called $a_n=\frac{a_0}{2^n(n!)}$ but if we call them "n" for now wouldn't that conflict because Let's say for n=5 $\frac{a_0}{2^3(3!)}$ which is not true (I'm referring to y'=xy, the first problem)

82. MathSofiya

Well it's what we called the pattern initially before writing it in terms of k's

83. MathSofiya

ok found the laundry card brb

84. TuringTest

The pattern is not in terms of n like that, if it were it would be$a_n=\frac{a_0}{2^{n/2}(\frac n2!)}$for even n subscripts of a, and$a_n=0$ for all odd n subscripts of a. I think a major problem here is that we are getting confused with the n in the $$n=...$$ that we are using to plug in to find the particular constant $$a_n$$, and the n in the $$a_n$$ in the constant. Those two n's are not equal! What exactly is your argument that in the last problem you are referring too we do no not have that$n=5:a_6=\frac{a_4}6=\frac{a_0}{2\cdot4\cdot6}=\frac{a_0}{2^3(3!)}$??

85. MathSofiya

Ok that makes sense. I thought the n's were the same. So the n's are just a variable to hold place until we change the argument to k's I guess. wait, what do you mean by the last sentence? so the n=5 statement above is not accurate?

86. TuringTest

yes it is accurate, plugging in n=5 into the recursion formula$a_{n+1}=\frac{a_{n-1}}{n+1}$gives$n=5:a_6=\frac{a_4}6=\frac{a_0}{2\cdot4\cdot6}=\frac{a_0}{2^3(3!)}$the $$n=...$$ is just what we use to get an expression relating two other constants $$a_{n+ p}$$and $$a_{n+ q}$$ where p and q are some number (in this case above p=-1 and q=1) so the n we plug in to test is $$not$$ the same as the n in the subscripts, because the subscripts in our recursion relation are usually not just $$a_n$$ (the same n as we are plugging in for the test) but really $$a_{n+p}$$

87. TuringTest

We want our expression for each constant in terms of its $$subscript$$, not the $$n=...$$ we use to test the relation, hence we divide the subscripts into even and odd to make the pattern more clear. Once we notice that by doing that we can get meaningful expressions for each constant a in terms of the subscript (however we write) that is what we use...

88. TuringTest

in this case by dividing the subscripts into those of the form 2k and those of the form 2k+1 we get a new way to write each constant in terms of k

89. TuringTest

...and whatever base constant we can't determine, like $$a_0$$ which we do not know since we are not given the initial conditions

90. MathSofiya

ok that makes more sense now. Let's see if I can apply it to this problem now.

91. TuringTest

go for it, I'm gonna get a few empanadas for breakfast :) (sortof like Mexican hot-pockets :) brb

92. MathSofiya

sounds good

93. MathSofiya

Let's see here.... $a_2=\frac{a_0}{2!}$ $a_3=\frac{a_1}{3!}$ $a_4=\frac{a_0}{4!}$ $a_5=\frac{a_1}{5!}$ $a_2k=\frac{a_0}{k!}$ my way of saying if the subscript is even...

94. MathSofiya

$a_{2k+1}=\frac{a_1}{k!}$

95. TuringTest

is it? what is$$a_3$$ ?

96. MathSofiya

oh my 2k+1=3 2k=2 k=1

97. MathSofiya

yeah I'm wrong

98. MathSofiya

3k?

99. MathSofiya

your patients with me is truly astonishing. I would have strangled the student.

100. TuringTest

haha, but I haven't done this in a long time either, so it's a good review for me too.

101. MathSofiya

*patience

102. TuringTest

$a_n=\frac{a_n}{(n+1)(n+2)}$$n=0:a_2=\frac {a_0}{2}=\frac{a_0}{2!}$$n=1:a_3=\frac {a_1}{6}=\frac{a_1}{3\cdot 2}=\frac{a_1}{3!}$$n=2:a_4=\frac {a_2}{12}=\frac{a_0}{4\cdot3\cdot2\cdot1}=\frac{a_0}{4!}$$n=3:a_5=\frac {a_3}{20}=\frac{a_3}{10\cdot2}=\frac{a_1}{5\cdot4\cdot3\cdot2\cdot1}=\frac{a_1}{5!}$now hopefully you have already noticed that the denominator on each $$a_n$$ is $$n!$$$a_n=\frac{a_0}{n!}$if n is even, and$a_n=\frac{a_1}{n!}$if n is odd given that, simply rewrite the subscripts as 2k or 2k+1 depending on whether they are even or odd, and then sub in for n in the general expressions

103. MathSofiya

yep, I honestly saw that relationship all along, I just didn't know how to express that in k's $a_{2k}=\frac{a_0}{2k}$ $a_{2k+1}=\frac{a_0}{2k+1}$ sorry I kinda feel silly now

104. MathSofiya

way easy

105. TuringTest

you forgot the factorials ;)

106. MathSofiya

!!!!!!!!!!!! there

107. MathSofiya

LOL ok let me rewrite them

108. TuringTest

and you forgot that for odd subscripts it is a_1 in the numerator

109. MathSofiya

$a_{2k}=\frac{a_0}{(2k)!}$ \a_{2k+1}=\frac{a_1}{2k+1}[\] I didn't forgot...I just checking if you were paying attention :P

110. MathSofiya

$a_{2k+1}=\frac{a_1}{2k+1}$

111. MathSofiya

oh my

112. TuringTest

haha :P

113. MathSofiya

$a_{2k+1}=\frac{a_1}{(2k+1)!}$

114. MathSofiya

happy?

115. MathSofiya

oh wait there is more...

116. TuringTest

Much better :) I wanna make a subtle point about the possible values of k we are talking about, but I will save it till the end so as not to confuse you.

117. MathSofiya

$y=\sum_{n=0}^\infty a_nx^n=a_0\sum_{k=0}^\infty\frac{x^{2k}}{(2k)!}$ $y=\sum_{n=0}^\infty a_nx^n=a_1\sum_{k=0}^\infty\frac{x^{2k+1}}{(2k+1)!}$

118. TuringTest

you've got the right idea, but we need to use superposition to get the full answer

119. TuringTest

if we accept only one answer or the other we are excluding either all the even or odd exponents, so how can we fix that?

120. MathSofiya

put them into one formula somehow

121. MathSofiya

oh y and c_1 and so on and so forth...hold on

122. TuringTest

superposition...

123. MathSofiya

124. TuringTest

yep :) c1=a0 c2=a1

125. MathSofiya

$c_1\sum_{k=0}^\infty\frac{x^{2k}}{(2k)!}+c_2\sum_{k=0}^\infty\frac{x^{2k+1}}{(2k+1)!}=0$ huh?

126. MathSofiya

gosh the sums probably shouldn't be there...

127. TuringTest

no I was just saying that the c1 you are used to is called a0 in this case, so...

128. TuringTest

$y=a_0\sum_{k=0}^\infty\frac{x^{2k}}{(2k)!}+a_1\sum_{k=0}^\infty\frac{x^{2k+1}}{(2k+1)!}=0$

129. TuringTest

wait, why =0 ?

130. TuringTest

$y=a_0\sum_{k=0}^\infty\frac{x^{2k}}{(2k)!}+a_1\sum_{k=0}^\infty\frac{x^{2k+1}}{(2k+1)!}$

131. MathSofiya

I just looked at the example in the book Uhm Ohhh,,, but they don't have to be written in c_1 and c_2 right?

132. MathSofiya

can I put my laundry in the dryer?

133. MathSofiya

brb

134. MathSofiya

2 mins

135. TuringTest

nah, constants are constants, you can call them whatever you want but I just wanted to keep our notation consistent, and since they are equivalent to c1 and c2 (constants that depend on the initial conditions) and yes, you have my permission to do your laundry :P

136. MathSofiya

ok I'm back

137. MathSofiya

yep makes sense

138. TuringTest

so review what we did, look again at example one one here: http://tutorial.math.lamar.edu/Classes/DE/SeriesSolutions.aspx and tell me if you have any questions

139. TuringTest

example 1 on*

140. MathSofiya

I read it, I don't think I have any questions. 22 hrs later I finally understand it (hopefully I'll be able to recreate it)

141. TuringTest

awesome! there is, as I said, a subtle point to be made about the possible values of k (as paul mentions) but since it really doesn't affect this problem I'll leave it for you to read, or we'll cross that bridge when we come to it :)

142. MathSofiya

sounds good...I'll do 2 problems on my own from beginning to end and we'll see if I make error or still struggle on some points here and there...

143. TuringTest

I do encourage you to read the whole page I linked you to though, to see some of the differences encountered in other problems. Good luck!

144. MathSofiya

145. TuringTest

nope, bought 'em at the corner I only cook hamburgers and pasta

146. MathSofiya

what? you've been in Mexica for a little while now! Gotta learn to cook like the locals!

147. TuringTest

well I make quesadillas, I guess that's a start lol

148. MathSofiya

yep, work your way up from here :P

149. TuringTest

:)

150. mahmit2012

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151. mahmit2012

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152. mahmit2012

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153. mahmit2012

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