Reverse continued fractions

Question

Basic continued fractions arise from recurrence relations such as:

$$ n = a + \frac{b}{n}, $$

This gives rise to the continued fraction:

$$ n = a + \frac{b}{a+\frac{b}{a+\frac{b}{a+...}}}. $$

What about relations such as:

$$ n = a + \frac{n}{b}? $$

Do these give rise to things like:

$$ n = a + \frac{a+\frac{a+\frac{a+\frac{a+\frac{a+...}{b}}{b}}{b}}{b}}{b}? $$

Of course, one could just solve the original equation as:

\begin{align} n &= a + \frac{n}{b} \\ n(1-b^{-1}) &= a \\ n &= \frac{a}{1-b^{-1}}. \end{align}

But couldn't the above "reverse continued fraction" be generalized like the usual one to create interesting structures and definitions of known constants? For instance, one can obtain this strange result for $n=a + \frac{n}{b²}$ by setting $a=1$ and $b=\sqrt{2}^{-1}$:

$$ -1 = 1 + \frac{1+\frac{1+\frac{1+\frac{1+\frac{1+...}{\sqrt{2}^{-1}}}{\sqrt{2}^{-1}}}{\sqrt{2}^{-1}}}{\sqrt{2}^{-1}}}{\sqrt{2}^{-1}}? $$

Is this known/of any particular interest?

Answer 1

Note that $$n = a + \frac{a+\frac{a+\frac{a+\frac{a+\frac{a+...}{b}}{b}}{b}}{b}}{b}=a+\frac ab + \frac a{b^2}+ \frac a{b^3} +\ldots$$ gives you a geometric series. If the constants vary it is still just another way of writing an infinite sum, so doesn't give anything new.

Answer 2

I see what Shark means: with an infinite periodic conventional c.f. you get a quadratic for the limiting value, but with a reverse c.f. you only get a linear equation.

Playing with your idea: Let $\pi = 3 + (1 + x_1)/n_1$ where $x_1 > 0$ and $n_1$ is an integer. The least $n_1$ that will do is $8$, so use that. Then express $x_1 = (1 + x_2)/n_2$, taking the least $n_2$, which happens to be 8 again. Proceed in this way and get

$$\pi = 3 + {1 + {1 + {1 +{1 + {{1 + \ldots}\over 300}\over 19}\over 17}\over 8}\over 8}$$

The sequence $3; 8, 8, 17, 19, 300, \ldots$ is non-decreasing, and this always happens after the first term (why?). For $\sqrt2$ we get $1; 3, 5, 5, 16, 18, 78, 102, 120, 144, \ldots$, hard to see a pattern there. For $e$ we get (ah!) $2; 2, 3, 4, 5, 6, 7, 8, 9, 10, \ldots$. But how to prove it?