Use a power series expansion about x=0 to find a general solution for:
(2x −3)y''−xy'+y =0
(Determine at least the first four nonzero terms.)
Assume that y = Σ(n = 0 to ∞) a(n) x^n.
Substituting this into the DE yields
(2x - 3)Σ(n = 2 to ∞) n(n-1) a(n) x^(n-2) - x Σ(n = 1 to ∞) n a(n) x^(n-1) + Σ(n = 0 to ∞) a(n) x^n = 0
Simplifying:
Σ(n = 2 to ∞) 2n(n-1) a(n) x^(n-1) - Σ(n = 2 to ∞) 3n(n-1) a(n) x^(n-2) - Σ(n = 1 to ∞) n a(n) x^n
+ Σ(n = 0 to ∞) a(n) x^n = 0
Re-indexing to x^n:
Σ(n = 1 to ∞) 2(n+1)n a(n+1) x^n - Σ(n = 0 to ∞) 3(n+2)(n+1) a(n+2) x^n -
Σ(n = 1 to ∞) n a(n) x^n + Σ(n = 0 to ∞) a(n) x^n = 0
Collecting like terms:
(a(0) - 6 a(2)) + Σ(n = 1 to ∞) [-3(n+2)(n+1) a(n+2) + 2n(n+1) a(n+1) - (n-1) a(n)] x^n = 0
Assuming that a(0) and a(1) are arbitrary,
a(0) - 6 a(2) = 0
==> a(2) = (1/6) a(0).
For n > 0, we have -3(n+2)(n+1) a(n+2) + 2n(n+1) a(n+1) - (n-1) a(n) = 0
==> a(n+2) = [2n(n+1) a(n+1) - (n-1) a(n)] / [3(n+2)(n+1)].
a(3) = 4 a(2)/18 = (1/27) a(0)
a(4) = [12 a(3) - a(2)] / 36 = (5/648) a(0), which also does not have a(1) in it (nor do any other coefficients).
So, a general solution is
y = a(0) (1 + (1/6)x^2 + (1/27)x^3 + (5/648)x^4 ...) + a(1) x.
I hope this helps!
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Verified answer
Assume that y = Σ(n = 0 to ∞) a(n) x^n.
Substituting this into the DE yields
(2x - 3)Σ(n = 2 to ∞) n(n-1) a(n) x^(n-2) - x Σ(n = 1 to ∞) n a(n) x^(n-1) + Σ(n = 0 to ∞) a(n) x^n = 0
Simplifying:
Σ(n = 2 to ∞) 2n(n-1) a(n) x^(n-1) - Σ(n = 2 to ∞) 3n(n-1) a(n) x^(n-2) - Σ(n = 1 to ∞) n a(n) x^n
+ Σ(n = 0 to ∞) a(n) x^n = 0
Re-indexing to x^n:
Σ(n = 1 to ∞) 2(n+1)n a(n+1) x^n - Σ(n = 0 to ∞) 3(n+2)(n+1) a(n+2) x^n -
Σ(n = 1 to ∞) n a(n) x^n + Σ(n = 0 to ∞) a(n) x^n = 0
Collecting like terms:
(a(0) - 6 a(2)) + Σ(n = 1 to ∞) [-3(n+2)(n+1) a(n+2) + 2n(n+1) a(n+1) - (n-1) a(n)] x^n = 0
Assuming that a(0) and a(1) are arbitrary,
a(0) - 6 a(2) = 0
==> a(2) = (1/6) a(0).
For n > 0, we have -3(n+2)(n+1) a(n+2) + 2n(n+1) a(n+1) - (n-1) a(n) = 0
==> a(n+2) = [2n(n+1) a(n+1) - (n-1) a(n)] / [3(n+2)(n+1)].
a(3) = 4 a(2)/18 = (1/27) a(0)
a(4) = [12 a(3) - a(2)] / 36 = (5/648) a(0), which also does not have a(1) in it (nor do any other coefficients).
So, a general solution is
y = a(0) (1 + (1/6)x^2 + (1/27)x^3 + (5/648)x^4 ...) + a(1) x.
I hope this helps!