grandes-ecoles 2020 Q33

grandes-ecoles · France · centrale-maths2__psi Sequences and series, recurrence and convergence Convergence proof and limit determination

We define a sequence $(a_n)_{n \geqslant 1}$ by setting $$\forall n \in \mathbb{N}^*, \quad a_n = \frac{(-n)^{n-1}}{n!}.$$ We define, when possible, $S(x) = \sum_{n=1}^{+\infty} a_n x^n$, with radius of convergence $R$. Let $W$ be the Lambert function defined in Part I (inverse of $f|_{[-1,+\infty[}$ where $f(x)=xe^x$). Using the results of Questions 31 and 32, deduce that $$\forall x \in ]-R, R[, \quad S(x) = W(x).$$

We define a sequence $(a_n)_{n \geqslant 1}$ by setting
$$\forall n \in \mathbb{N}^*, \quad a_n = \frac{(-n)^{n-1}}{n!}.$$
We define, when possible, $S(x) = \sum_{n=1}^{+\infty} a_n x^n$, with radius of convergence $R$. Let $W$ be the Lambert function defined in Part I (inverse of $f|_{[-1,+\infty[}$ where $f(x)=xe^x$). Using the results of Questions 31 and 32, deduce that
$$\forall x \in ]-R, R[, \quad S(x) = W(x).$$

Paper Questions

Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 Q21 Q22 Q23 Q24 Q25 Q26 Q27 Q28 Q29 Q30 Q31 Q32 Q33 Q34 Q35 Q36 Q37 Q38 Q39