Infinite Nested Radical

Question

Ramanujan discovered that $$x+n+a=\sqrt{ax+(n+a)^2+x\sqrt{a(x+n)+(n+a)^2+...}}$$ (see equation (27) here). I didn't understand how we can use this (basically what to put in place of $x, n, a$) to calculate the value of $$f(k)=\sqrt{k+2\sqrt{k+3\sqrt{k+4\sqrt{...}}}}$$ Also the page stated that the justification of this process in general (and in the particular example of $ln(\sigma)$, where $\sigma$ is Somos's quadratic recurrence constant) is given by Vijayaraghavan (in Ramanujan 2000, p. 348). I couldn't find 'Ramanujan 2000' anywhere so a source to that would also be helpful.