# Construct a function satisfied some conditions

• October 18th 2009, 06:49 AM
Xingyuan
Construct a function satisfied some conditions
Construct a function $f(x)\in C^{\infty}(-\infty,+\infty)$
that satisfy $f^{(2n+1)}(0) = n$ and $f^{(2n)}(0) = 0$,for all $n\geq0$.
Furthermore,show that there are infinity $f(x)$satisfy the condition above
(Angry)
• October 18th 2009, 10:40 AM
Opalg
Quote:

Originally Posted by Xingyuan
Construct a function $f(x)\in C^{\infty}(-\infty,+\infty)$
that satisfy $f^{(2n+1)}(0) = n$ and $f^{(2n)}(0) = 0$,for all $n\geq0$.
Furthermore,show that there are infinity $f(x)$satisfy the condition above
(Angry)

The function $\sum_{n=0}^\infty\frac{nx^{2n+1}}{(2n+1)!} = \tfrac12(x\cosh x-\sinh x)$ will have that property. So will any function obtained from that one by adding any multiple of a $C^\infty$-function whose derivatives at 0 are all 0, such as the function $f(x) = \begin{cases}e^{-1/x^2}&(x\ne0),\\0&(x=0).\end{cases}$