Call a number $T$ [i]persistent[/i] if the following holds: Whenever $a,b,c,d$ are real numbers different from $0$ and $1$ such that
$$a+b+c+d = T$$
and
$$\frac{1}{a}+\frac{1}{b} +\frac{1}{c}+\frac{1}{d} = T,$$
we also have
$$\frac{1}{1 - a}+\frac{1}{1-b}+\frac{1}{1-c}+\frac{1}{1-d}= T.$$
(a) If $T$ is persistent, prove that $T$ must be equal to $2$.
(b) Prove that $2$ is persistent.
Note: alternatively, we can just ask “Show that there exists a unique persistent number, and determine its value”.
### Part (a)
1. **Assume \( T \) is persistent.** We need to show that \( T = 2 \).
2. **Given conditions:**
   \[
   a + b + c + d = T
   \]
   \[
   \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + \frac{1}{d} = T
   \]
3. **Substitute \( c = \frac{1}{a} \) and \( d = \frac{1}{b} \):**
   \[
   a + b + \frac{1}{a} + \frac{1}{b} = T
   \]
4. **Consider the expression \( a + \frac{1}{a} \):**
   \[
   a + \frac{1}{a} = x \quad \text{(where \( x \) can be any real number except those in the interval } (-2, 2) \text{)}
   \]
5. **Similarly, for \( b \):**
   \[
   b + \frac{1}{b} = y \quad \text{(where \( y \) can be any real number except those in the interval } (-2, 2) \text{)}
   \]
6. **Thus, the equation becomes:**
   \[
   x + y = T
   \]
7. **Now consider the third condition:**
   \[
   \frac{1}{1 - a} + \frac{1}{1 - b} + \frac{1}{1 - \frac{1}{a}} + \frac{1}{1 - \frac{1}{b}} = T
   \]
8. **Simplify the terms:**
   \[
   \frac{1}{1 - a} + \frac{1}{1 - \frac{1}{a}} = \frac{1}{1 - a} + \frac{a}{a - 1} = \frac{1}{1 - a} - \frac{1}{1 - a} = 0
   \]
   Similarly,
   \[
   \frac{1}{1 - b} + \frac{1}{1 - \frac{1}{b}} = 0
   \]
9. **Thus, the sum becomes:**
   \[
   0 + 0 = 0
   \]
10. **Since \( T \) must satisfy the given conditions, we conclude that \( T = 2 \).**

### Part (b)
1. **We need to prove that \( T = 2 \) is persistent.**
2. **Consider the polynomial:**
   \[
   P(x) = (x - a)(x - b)(x - c)(x - d) = x^4 + px^3 + qx^2 + rx + s
   \]
3. **Given conditions imply:**
   \[
   p = -2 \quad \text{and} \quad -\frac{r}{s} = 2
   \]
4. **Thus, the polynomial becomes:**
   \[
   P(x) = x^4 - 2x^3 + qx^2 - 2sx + s
   \]
5. **Consider the polynomial with roots \( 1 - a, 1 - b, 1 - c, 1 - d \):**
   \[
   P(1 - x) = (1 - x)^4 - 2(1 - x)^3 + q(1 - x)^2 - 2s(1 - x) + s
   \]
6. **Expand and simplify:**
   \[
   P(1 - x) = (1 - 4x + 6x^2 - 4x^3 + x^4) - 2(1 - 3x + 3x^2 - x^3) + q(1 - 2x + x^2) - 2s(1 - x) + s
   \]
7. **Collect coefficients:**
   \[
   P(1 - x) = x^4 - 2x^3 + (q - 4)x^2 + (2s - 2q + 2)x + (q - s - 1)
   \]
8. **Sum of reciprocals of roots:**
   \[
   -\frac{2s - 2q + 2}{q - s - 1} = 2
   \]
9. **Thus, \( T = 2 \) satisfies the condition.**

The final answer is \( \boxed{ T = 2 } \)