Kansas State University

STEP 1: Plug in $\displaystyle y(x) = \sum_{n=0}^{\infty}a_n x^n $ and compute all the different terms in the equation $$ \begin{align} (6x^2 + 5)y &= \sum_{ n = 0 }^{ \infty }\,6 a_{ n } x^{ n+2 } + \sum_{ n = 0 }^{ \infty }\,5 a_{ n } x^{ n }\\ -7y' &= \sum_{ n = 1 }^{ \infty }\,-7n a_{ n } x^{ n-1 }\\ (-3x^2 + 6)y'' &= \sum_{ n = 2 }^{ \infty }\,-3n(n-1) a_{ n } x^{ n } + \sum_{ n = 2 }^{ \infty }\,6n(n-1) a_{ n } x^{ n-2 } \end{align} $$ STEP 2: Make the substitutions $ k = n-2 $, $ j = n-1 $ and $ q = n+2 $ to make all terms of the form $ x^{\text{index}} $ rather than the $ x^{\text{index}-1} $ or $ x^{\text{index}+2} $ or whatever. $$ \begin{align} (6x^2 + 5)y&= \sum_{ q = 2 }^{ \infty }\,6 a_{ q-2 } x^{ q } + \sum_{ n = 0 }^{ \infty }\,5 a_{ n } x^{ n }\\ -7y'&= \sum_{ j = 0 }^{ \infty }\,-7(j+1) a_{ j+1 } x^{ j }\\ (-3x^2 + 6)y''&= \sum_{ n = 2 }^{ \infty }\,-3n(n-1) a_{ n } x^{ n } + \sum_{ k = 0 }^{ \infty }\,6(k+2)(k+1) a_{ k+2 } x^{ k }\end{align} $$ STEP 3: Change all the indices to the same letter (I use $ m $) and plug into the equation. $$ \begin{align} (-3x^2 + 6)y'' - 7y' + (6x^2 + 5)y &= \sum_{ m = 2 }^{ \infty }\,-3m(m-1) a_{ m } x^{ m } + \sum_{ m = 0 }^{ \infty }\,6(m+2)(m+1) a_{ m+2 } x^{ m } \\ &+ \sum_{ m = 0 }^{ \infty }\,-7(m+1) a_{ m+1 } x^{ m } \\ &+ \sum_{ m = 2 }^{ \infty }\,6 a_{ m-2 } x^{ m } + \sum_{ m = 0 }^{ \infty }\,5 a_{ m } x^{ m } \end{align} $$ STEP 4: Collect like terms. $$ \begin{align} &(12a_2 - 7a_1 + 5a_0) + (36a_{3} - 14a_{2} + 5a_{1})x \\ &+ \sum_{ m = 2 }^{ \infty }\,(6(m+2)(m+1)a_{m+2} - 3m(m-1)a_m - 7(m+1)a_{m+1} + 5a_m + 6a_{m-2})x^m = 0 \end{align} $$ STEP 5: Equate coefficients to 0.

Equating the constant term to 0 we get $$ a_2 = \frac{7a_1 - 5a_0}{12} $$ Equating the linear term to 0 we get $$ a_3 = \frac{14a_2 - 5a_1}{36} $$ Finally, equating the general term to 0, we find that for $ m \ge 2,$ $$ a_{m+2} = \frac{7(m+1)a_{m+1} - (-3m(m-1) + 5)a_m - 6a_{m-2}}{6(m+2)(m+1)} $$ STEP 6: We know that $ a_0 = y(0) = -1 $ and $ a_1 = y'(0) = -8.$ We then plug these values into the formulas found in step 5 to compute the coefficients of the solution.

From the equation for the constant term we get $$ a_2 = \frac{7(-8) - 5(-1)}{12} = -17/4 $$ From the equation for the linear term we get $$ a_3 = \frac{14(-17/4) - 5(-8)}{36} = -13/24 $$ Finally, using the recurrence equation with $ m = 2 $ we get $$ a_4 = \frac{7(2+1)(-13/24) - (-32(2-1) + 5)(-17/4) - 6(-1)}{6(4)(3)} = -77/576 $$

So our solution is $$ y(x) = -1 - 8x - (17/4)x^2 - (13/24)x^3 - (77/576)x^4 + \cdots $$

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