# Are these continued fractions of integrals known?

Motivated by Bowman and McLaughlin (2000) on polynomial continued fractions, I considered an extension to functional continued fractions, where the numerator of each fraction is the integral of the previous fraction.

That is, define the continued fraction integral transform (CFIT) as \begin{align}\{\mathcal If(t)\}(s)=\dfrac{f(s)}{1+\dfrac{\int_0^s f(x)\,dx}{1+\dfrac{\int_0^s\int_0^u f(x)\,dx\,du}{1+\cdots}}}=\dfrac{f(s)}{1+\dfrac{f^{(-1)}(s)}{1+\dfrac{f^{(-2)}(s)}{1+\cdots}}}={\large{\mathop{\mathrm{K}}_{n=0}^{\infty}}}\frac{f^{(-n)}(s)}1\end{align} where $$f\in C^\infty(\Bbb R)$$ and $$f^{(-k)}$$ is Lagrange's notation for the $$k$$th antiderivative of $$f$$.

Some elementary properties of the transform include

1. $$\{\mathcal I(0)\}(s)=0$$;

2. $$\{\mathcal If\}(0)=f(0)$$;

3. $$\{\mathcal If\}(s)=\dfrac{f'(s)}{\{\mathcal If'\}(s)}-1$$ for all non-constant $$f$$.

Considering the simplest, non-trivial case $$f(t)\equiv1$$, we have $$f^{(-k)}(t)=t^k/k!$$ so that $$\{\mathcal I(1)\}(s)=\dfrac{1}{1+\dfrac s{1+\dfrac{s^2/2!}{1+\cdots}}}.$$

We can see that as $$n$$ increases, $$\mathcal I(1)$$ produces more waves. As @PeterForeman mentioned in the comments, the limit $$\lim\limits_{s\to-\infty}\mathcal I(1)$$ does not exist since it is zero for odd $$n$$ and $$+1$$ for even $$n$$. However, I believe there are more things that can be investigated.

Questions

1. Has the CFIT already been studied/discovered? If so, reference to literature would be appreciated.

2. Going back to the example of $$\{\mathcal I(1)\}(s)$$, is it true that the maximum value of each wave increases as $$s$$ increases up to the first wave? Are there an infinite number of such waves, and is there a closed form for $$\max\mathcal I(1)$$?

• @TheSimpliFire Also $\lim_{s\to-\infty}I(1)$ clearly does not exist as it equals $0$ for any odd number of terms in an approximation and $1$ for any even number. Jul 6, 2019 at 20:16

Not an answer, but one more picture, with the functions for 100 and 150 terms superposed.

It is not clear what is happening on the left.

EDIT:

Here is the code (using sagemath):

def fun(x, N=150):
L = [x**k/gamma(k + 1) for k in range(N, -1, -1)]
resu = 1.0
for t in L:
resu = t / (1 + resu)
return resu
P1 = plot(lambda x:fun(x, 100),-12, 3, aspect_ratio=1, color='red'); P1