Past exam of the mathematics course of the year 1 of the natural sciences course of the University of Cambridge / 2016 / Paper 1 / 15 / b / ii

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- Solution ii

Solution

Since

a_{1} = f^{'} (0) \neq = 0

, substitute the proposed inverse series into the forward one. Comparing coefficients of

f, f^{2}, f^{3}

gives

a_{1} b_{1} = 1, a_{1} b_{2} + a_{2} b_{1}^{2} = 0, a_{1} b_{3} + 2 a_{2} b_{1} b_{2} + a_{3} b_{1}^{3} = 0.

Solving successively yields the cubic series reversion coefficients

b_{1} = \frac{1}{a _{1}}, b_{2} = - \frac{a _{2}}{a _{1}^{3}}, b_{3} = \frac{2 a _{2}^{2} - a _{1} a _{3}}{a _{1}^{5}} .

The conditions

f (0) = 0

and

a_{1} \neq = 0

ensure that both local series are centred at zero and that the inverse has a linear leading term.

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