# Fixed Point: Finding rate of convergence

I have $g(x) = \sqrt{1+\log(x)}$, I want to find the rate of convergence using fixed point iteration. I have confirmed that this is linearly convergent, because the absolute value of its derivative is less than $1$, but I want to know how fast it converges to $1$ (which is our fixed point).

How can I find the rate of convergence for : $x_{i+1} = \sqrt{1+\log(x_i)}$?

I have tried squaring both sides but wasn't able to weasel out a relationship between $x_{i+1}$ and $x_i$.

If you are near a root $r$ of $x-g(x)=0$ then let $x_n=r+\epsilon_n$. The linear approximation of the next iterate is $$x_{n+1}=r+\epsilon_{n+1}=g(x_n)=g(r+\epsilon_n)\approx g(r)+\epsilon_ng^{\prime}(r)=r+\epsilon_ng^{\prime}(r)$$ So the error $\epsilon$ just gets multiplied by $g^{\prime}(r)$ at each iteration with the result that $$\epsilon_n\approx\epsilon_0\left(g^{\prime}(r)\right)^n$$ Near the fixed point $r\approx0.450763652$, $$g^{\prime}(r)=\frac1{2r\sqrt{1+\ln r}}=\frac1{2r^2}=2.460776817>1$$ so we won't converge there. Near $r=1$, $g^{\prime}(r)=\frac12$ so $\epsilon_n\approx\frac{\epsilon_0}{2^n}$ provided our initial aproximation was close enough to $1$.