In an old text I am reading I have encountered something that looks like a two-dimensional Riemann-Stieltjes integral. I am wondering if there are any good books or texts on this subject?
Update: It is requested that I write the integral. There are a lot of details here, I don't need help with the details I only need help with the request of sources.
The Riemann sum is:
$\sum\limits_{r=1}^n\sum\limits_{m=1}^{n'}v^{s_r+s_m'-2s}\Gamma_{\nu jk}(s,s_r,s_m')E_{\nu s}\{[Y(s,t_r)-Y(s,t_{r-1})][Y(s,t_{m'})-Y(s,t_{m-1}')]\}$.
It is said that this converges to: $\int_{\Omega}v^{t+\tau-2s}\Gamma_{\nu j k}(s,t,\tau)d_{(t,\tau)}r_{vs}(t,\tau)$. Where $\Omega$ is a certain subset of $\mathbb{R}^2$. It is also said that
$\frac{\partial^2 }{\partial t \partial \tau}r_{\nu s}(t,\tau)=P_{\nu j}(s,t)\mu_{jk}(t)P_{kj}(t,\tau)\mu_{jk}(\tau)$.
Then it said that the integral above equals: $\int_\Omega v^{t+\tau-2s}\Gamma_{vjk}(s,t,\tau)P_{\nu j}(s,t_1(t,\tau))\mu_{jk}(t_1(t,\tau))P_{kj}(t_1(t,\tau),t_2(t,\tau))\mu_{jk}(t_2(t,\tau))d(\tau,t)$.
Where $t_1,t_2$ are min and max functions, $\Gamma$ is a given function.