Proof of Feller condition for CIR square root process. Any reference?

Question

Could you please give me some reference for the proof of the so-called Feller condition as to a stochastic differential equation of the form: $$dr_t=a(b-r_t)dt+\sigma\sqrt{r_t}dB_t\tag{1}$$ with $\left(B_t\right)_{t\geq0}$ denoting a Brownian motion on the filtered probability space $\left(\Omega,\mathcal{F},\mathcal{F}_n,\mathbb{P}\right)$?

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I found something here, but I cannot really understand it, hence I am searching for something alternative. Is there some alternative proof (e.g. from a book)?

Answer 1

It is covered very nicely in Iain Clark's Foreign Exchange Option Pricing, A Practitioner’s Guide (pages 98-104). The book also contains references to the relevant literature including Feller's original paper.

Answer 2

I believe your SDE has an unintended error. It should be:

$$ dr_t = a \cdot (b - r_t) \cdot dt + \sigma \cdot \sqrt{r_t} \cdot dB_t. $$

On the other hand, the Feller condition is discussed and explained in Section 10.2.1.2 (pg. 432) of the Andersen and Piterbarg book: Interest Rate Modeling.

Hope it helps!