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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

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  • 5 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...

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