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How to find Mclaurins series which satisfy differential equations?
I need to find the mclaurines series for y(x) satisfying y'= y^2 +xy +1 when y(0)= 1
1 Answer
- MorewoodLv 75 years ago
Pretty much plug and chug:
McLaurin series: y(x) = ∑ₙ (aₙ xⁿ), summed on "n" from zero to infinity.
Substituting x=0 in y(0)=1 gives a₀=1.
Then, computing each term in the given equation:
y'(x) = ∑ₙ ((n+1)aₙ₊₁ xⁿ)
x×y(x) = ∑ₙ (aₙ₋₁ xⁿ)
y(x)² = ∑ₙ ((∑ₖ aₖaₙ₋ₖ) xⁿ), summed on "k" from 0 to "n".
1 = ∑ₙ (δ₀ₙ xⁿ), where δ₀₀=1 and, for n≠0, δ₀ₙ=0.
Shows the given equation as:
∑ₙ ((n+1)aₙ₊₁ xⁿ) = ∑ₙ (((∑ₖ aₖaₙ₋ₖ) + aₙ₋₁ + δ₀ₙ) xⁿ)
Equate like powers of x produces the recursion:
aₙ₊₁ = ((∑ₖ aₖaₙ₋ₖ) + aₙ₋₁ + δ₀ₙ) / (n+1), for n≥0 {and a₋₁=0}
Substituting n=0 and a₀=1 gives a₁=2, then substituting that gives a₂=2.5, then ...
So the required McLaurin series looks like:
y(x) = 1 + 2x + 2½x² + 3⅔x³ + ...
Not a function I recognize...