Feller condition may be violated, implying that ${\mathbb{Q}}^1$ may not be equivalent to ${\mathbb{P}}$.

2.  For the putative risk neutral measure ${\mathbb{Q}}^2$ we have

    1.  \[lemma2:point21\] If $\lambda(t)=a\sqrt{V(t)}$, $a\in{\mathbb{R}}$, then the drift of the variance process $V$ under ${\mathbb{Q}}^2$ equals $$\begin{aligned}
        \kappa\left(\theta-V(t)\right)-\sigma\rho (a-1)V(t)\end{aligned}$$ and the Feller condition is always satisfied under ${\mathbb{Q}}^2$, which is then a true equivalent martingale measure.

    2.  \[lemma2:point22\] If $\lambda(t) = a\sqrt{V(t)}+ \frac{b}{\sqrt{V(t)}}$, $a,b\in{\mathbb{R}}$, then the drift of the variance process $V$ under ${\mathbb{Q}}^2$ equals $$\begin{aligned}
        \kappa\left(\theta-V(t)\right)-\sigma\sqrt{V(t)}\rho\left((a-1)\sqrt{V(t)}+ \frac{b}{\sqrt{V(t)}}\right)\end{aligned}$$ and the Feller condition may be violated, implying that ${\mathbb{Q}}^2$ would be in such case not equivalent to ${\mathbb{P}}$.

    3.  \[lemma2:point23\] If $\lambda(t) = \frac{b}{\sqrt{V(t)}}$, $b\in{\mathbb{R}}$, then the drift of the variance process $V$ under ${\mathbb{Q}}^2$ is $$\begin{aligned}
        \kappa\left(\theta-V(t)\right)-\sigma\rho b +\sigma\rho V(t)\end{aligned}$$ and the Feller condition may be violated, implying that ${\mathbb{Q}}^2$ would be in such case not equivalent to ${\mathbb{P}}$.

The proof of these statements is straightforward using our previous notation and relationships and, therefore, omitted.

Formal Presentation of the 4/2 Model {#sec:modelPresentation}
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To provide in the case of more than two currencies a concrete specification of the market prices of risk, we proceed to introduce the ${\mathbb{R}}^d$-valued nonnegative stochastic process, called the volatility factor process, 