- #1
Gogeta007
- 23
- 0
Homework Statement
y'' + x2y' + xy = 0
Homework Equations
using power series:
y'' = [tex]\Sigma[/tex]cnn(n-1)xn-2 (n = 2 -> infinity)
y' = [tex]\Sigma[/tex]cn(n-1)xn-1 (n = 1 -> infinity)
y = [tex]\Sigma[/tex]cnxn (n = 0 -> infinty)
The Attempt at a Solution
by setting the above equation with its corresponding sumation formulas I get the following:
[tex]\Sigma[/tex]cnn(n-1)xn-2 + [tex]\Sigma[/tex]cn(n-1)xn+1 + [tex]\Sigma[/tex]cnxn+1
to make all the x's start at the same power I pull out 2 terms from the first sumation and 1 term from the last eq. so all x's start at the power of 2.
this yields:
2c2 + 6c3x + [tex]\Sigma[/tex]n=4cnn(n-1)xn-2 + [tex]\Sigma[/tex]cn(n-1)xn+1 + c0x + [tex]\Sigma[/tex]n=1cnxn+1
on the first sumation k=n-2, so n= k+2, on the second sumation k=n+1 and n=k-1 and the same for the third sumation.
substituting k's in their respective sumations we get:
c0x+2c2+6c3x + [tex]\Sigma[/tex]k=2 xk { ck+2(k+2)(k+1) + ck-1(k-1) = ck-1
from the last two terms we can factor a ck-1 and cancel out the 1's left leaving us only with kck-1
===========
from the identity that the coefficients should add up to zero, we conclude that c2=0 and c3= -c0/6
for the sumation part, I solve for c_k+2 = -{ck-1(k)}/(k+2)(k+1)
and plugging in values of k (1,2,3,4,5,6,7,8,9,10) I can see that k=3 = 0, k=6 = 0, k=9 = 0
for the other values I get sumations with c0 and sumations with c1
Im writing them the way I saw the teacher writing them (since the book does without the multiplication symbol (big pi))
I get the following:
c0 [tex]\Sigma[/tex]n=?? { (-1)n [tex]\Pi[/tex]m=0n-1(3m+1) } / 12*11*9*8*6*5*3*2
where big pi starts at m =0 and ends at m = n-1
for c_0
and for c_1
c1 [tex]\Sigma[/tex]n=?? { (-1)n [tex]\Pi[/tex]m=0n-1(3m+2) } / 10*9*7*6*4*3
I couldn't find a pattern for the denominators. . . I am thinking its two patterns ( for even and odds). . .anyways I'll do that later, my question is the following:
How do I know where to start the sumations of sigma in the answer, you know n=something ?? (why?)
in the book it says that the c0 sumation has the x3,x6,x9. . . and that c1 sumation has the x4,x7,x10
how do I know which sumation has what powers of x?
Some of the answers (in class) start with 1 + [tex]\Sigma[/tex]. . . or with x + [tex]\Sigma[/tex]. . .
how do I know when to pull out the first term? and again, on what n to start for the sumation?
Thanks a lot for reading trough this and thanks in advance for your help.