integrating with multiple indicator functions depending on each other

Question

I need to compute the following integral with three indicator functions, $I(x)$, $I_1(x_1)$ and $I_2(x_2)$,

$$ \int_0^1\int_0^1I(x)\left( \ I_1(x_1)\,x_{1} + I_2(x_2)\,x_{2}-1 \right) dx_1 dx_2 $$

where

$$ I(x)= \begin{cases} 1 & \text{if } I_1(x_1)x_1 + I_2(x_2)x_2 \geq s \\ 0 & \text{otherwise}% \end{cases} $$

$$ I_i(x_i)=% \begin{cases} 1 & \text{if } x_i \geq t_i \\ 0 & \text{otherwise} \end{cases} \qquad i=1,2 $$

I try to write it as

Integrate[
(x1 + x2 - 1)*(Boole[x1 + x2 >= s && x1 >= t1 && x2 >= t2]),{x1,0,1},{x2,0,1}]

It does generate some superficially long output, which makes me further suspect the syntax.

I found at this place a similar problem about multiple indicator functions was also discussed , however, it did not include the above "compound" indicator case.

PS: The original integral is with $n+1$ indicator functions. I simplified it to the above $2+1$ indicator functions with $\textit{U}(0,1)$ case.

Answer 1

How about adding some assumptions (I think the following is reasonable):

res = Integrate[(x1 + x2 - 1)*(Boole[
      x1 + x2 >= s && x1 >= t1 && x2 >= t2]), {x1, 0, 1}, {x2, 0, 1}];

FullSimplify[res, 0 < s < 1 && 0 < t1 <= t2 < 1]

$$\begin{cases} \frac{1}{2} (\text{t1}-1) (\text{t2}-1) (\text{t1}+\text{t2}) & s\leq \text{t1}\lor s\leq \text{t1}+\text{t2} \\ \frac{1}{6} \left(-2 s^3+3 s^2 (\text{t1}+\text{t2}+1)\\\qquad -6 s (\text{t1}+\text{t2})-\text{t1}^3+3 (\text{t1}+\text{t2})-\text{t2}^3\right) & \text{True} \end{cases}$$

A faster approach (if you already know what constraints to assume) is this:

res = Assuming[0 < s < 1 && 0 < t1 <= t2 < 1, 
  FullSimplify@
   Integrate[(x1 + x2 - 1)*(Boole[
       x1 + x2 >= s && x1 >= t1 && x2 >= t2]), {x1, 0, 1}, {x2, 0, 1}]]

which gives the same result.