I need to calculate a taylor polynomial for a function $f:\mathbb{R} \to \mathbb{R}$ where we know the following f\text{ }''(x)+f(x)=e^{-x} \text{ } \forall x $f(0)=0$ f\text{ }'(0)=2
How would I even start?
I need to calculate a taylor polynomial for a function $f:\mathbb{R} \to \mathbb{R}$ where we know the following f\text{ }''(x)+f(x)=e^{-x} \text{ } \forall x $f(0)=0$ f\text{ }'(0)=2
How would I even start?
The Frobenius method for solving differential equations is easily done: assume an ansatz
$f(x)=c_0+\sum_{k=1}^\infty c_k x^k$
and you have the derivatives
$\begin{align*} f^\prime(x)&=\sum_{k=1}^\infty k c_k x^{k-1}=c_1+\sum_{k=1}^\infty (k+1)c_{k+1} x^k\\ f^{\prime\prime}(x)&=\sum_{k=1}^\infty k(k+1)c_{k+1} x^{k-1} \end{align*}$
From these, you have $c_0=0$ and $c_1=2$ (why?); a relation for the other $c_k$ can be derived by comparing the series coefficients of $f(x)+f^{\prime\prime}(x)$ with the coefficients of $\exp(-x)$.
We have the following
$f''(x) + f(x) = e^{-x}$ and $f(0) = 0$, $f'(0) = 2$.
And thus we need to find $f^{(n)}(0)$ to construct the Taylor series.
Note that we already have two values and can find $f''(0)$ since
$f''(0) + f(0) = e^{-0}$
$f''(0) +0 = 1$
$f''(0) = 1$
So now we differentiate the original equation and get:
$f'''(x) + f'(x) = -e^{-x}$
But since we know $f'(0) = 2$, then
$f'''(0) + f'(0) = -e^{-0}$
$f'''(0) + 2 = -1$
$f'''(0) = -3$
And we have our third value. Differentiating one more time gives
$f^{IV}(x) + f''(x) = e^{-x}$
So again we have
$f^{IV}(0) + f''(0) =1$
$f^{IV}(0) + 1 =1$
$f^{IV}(0) =0$
Using this twice more you'll get
$f^{V}(0) =2$ $f^{VI}(0) =1$ $f^{VII}(0) =-3$
In general the equation is saying that
$f^{(2n+2)}(0) + f^{(2n)}(0) = 1$
$f^{(2n+1)}(0) + f^{(2n-1)}(0) = -1$
which will allow you to get all values.
A little summary of the already known values:
$f(0) = 0$
$f'(0) = 2$
$f''(0) = 1$
$f'''(0) = -3$
$f^{IV}(0) = 0$
$f^{V}(0) = 2$
$f^{VI}(0) = 1$
$f^{VII}(0) = -3$
Do you see a pattern?
Hint: Assume that $ f(x)=\sum\limits_{n\geqslant0}a_n\frac{x^n}{n!},\qquad g(x)=\sum\limits_{n\geqslant0}b_n\frac{x^n}{n!}, $ and that g=f''+f. Then f''(x)=\sum\limits_{n\geqslant0}a_{n+2}\frac{x^n}{n!}, hence, for every $n\geqslant0$, $b_n$ is...
To do: Identify $b_n$ for every $n\geqslant0$ and translate the initial conditions on $f$ in terms of $a_0$ and $a_1$.
This is a 2nd order linear non-homogeneous ODE with solution $f(x)=\frac{-1}{2}\cos x + \frac{5}{2} \sin x + \frac{1}{2} e^{-x}$. With the Taylor series for $\sin(x),\cos(x),$ and $e^{-x}$ in hand, you should be able to compute the series for $f$ straightforwardly.