Consider polynomial functions $ax^2 -bx +c$ with integer coefficients which have two distinct zeros in the open interval $(0,1).$ Exhibit with proof the least positive integer value of $a$ for which such a polynomial exists.
1. **Identify the conditions for the polynomial to have two distinct zeros in the interval \((0,1)\):**

   Let the polynomial be \( f(x) = ax^2 - bx + c \) with integer coefficients \(a\), \(b\), and \(c\). Suppose the polynomial has two distinct zeros \(0 < x_1 < x_2 < 1\). The vertex of the parabola, given by \( x_m = \frac{x_1 + x_2}{2} \), must lie in the interval \((0,1)\). The vertex formula for a quadratic function \(ax^2 - bx + c\) is \( x_m = \frac{b}{2a} \).

   Therefore, we have:
   \[
   0 < \frac{b}{2a} < 1 \implies 0 < b < 2a \implies 1 \leq b \leq 2a - 1
   \]

2. **Express \(b\) in terms of \(a\):**

   Let \( b = a + k \) where \( k \in \mathbb{Z} \) and \( |k| \leq a - 1 \).

3. **Determine the conditions for \(c\):**

   - For \(f(0) > 0\):
     \[
     c > 0 \implies c \geq 1
     \]

   - For \(f(1) > 0\):
     \[
     a - b + c > 0 \implies c \geq b - a + 1 \implies c \geq k + 1
     \]

4. **Ensure the discriminant is positive:**

   The discriminant \(\Delta\) of the quadratic equation \(ax^2 - bx + c = 0\) must be positive for the polynomial to have two distinct real roots:
   \[
   \Delta = b^2 - 4ac > 0
   \]

5. **Combine the conditions:**

   - From \(b = a + k\) and \(c \geq \max(1, k + 1)\):
     \[
     \Delta = (a + k)^2 - 4ac
     \]

   - Using \(c \geq 1\):
     \[
     \Delta \leq (a + k)^2 - 4a \cdot 1 = (a + k)^2 - 4a
     \]

   - Using \(c \geq k + 1\):
     \[
     \Delta \leq (a + k)^2 - 4a(k + 1) = (a + k)^2 - 4ak - 4a = (a - k)^2 - 4a
     \]

   Therefore:
   \[
   \Delta \leq \min((a + k)^2 - 4a, (a - k)^2 - 4a)
   \]

6. **Evaluate for small values of \(a\):**

   - For \(a = 1, 2, 3, 4\):
     \[
     \Delta \leq a^2 - 4a \leq 0 \quad \text{(no positive discriminant)}
     \]

   - For \(a = 5\):
     \[
     \Delta \leq (5 - |k|)^2 - 20
     \]
     The only possibility for \(\Delta > 0\) is \(k = 0\):
     \[
     b = a + k = 5, \quad c = 1
     \]

7. **Verify the polynomial:**

   The polynomial \(f(x) = 5x^2 - 5x + 1\) has the discriminant:
   \[
   \Delta = 5^2 - 4 \cdot 5 \cdot 1 = 25 - 20 = 5 > 0
   \]
   The roots are:
   \[
   x_{1,2} = \frac{5 \pm \sqrt{5}}{10}
   \]
   Both roots lie in the interval \((0,1)\).

Conclusion:
The minimum possible positive integer value of \(a\) is \(5\).

The final answer is \(\boxed{5}\)