Let $A, B$ be two variable points on a parabola $P_{0}$, such that the tangents at $A$ and $B$ are perpendicular to each other. Show that the locus of the centroid of the triangle formed by $A,B$ and the vertex of $P_0$ is a parabola $P_1 .$ Apply the same process to $P_1$ and repeat the process, obtaining the sequence of parabolas $P_1, P_2 , \ldots, P_n$. If the equation of $P_0$ is $y=m x^2$, find the equation of $P_n .$
1. **Parametrize the Parabola \( P_0 \):**
   The given parabola is \( y = mx^2 \). We can parametrize this curve by letting \( A = (t, mt^2) \) and \( B = (u, mu^2) \).

2. **Find the Slopes of the Tangents:**
   The slope of the tangent at \( A \) is given by the derivative of \( y = mx^2 \) at \( x = t \):
   \[
   \frac{dy}{dx} = 2mt
   \]
   Similarly, the slope of the tangent at \( B \) is:
   \[
   \frac{dy}{dx} = 2mu
   \]

3. **Condition for Perpendicular Tangents:**
   The tangents at \( A \) and \( B \) are perpendicular if the product of their slopes is \(-1\):
   \[
   (2mt)(2mu) = -1 \implies 4m^2tu = -1 \implies tu = -\frac{1}{4m^2}
   \]

4. **Coordinates of the Centroid:**
   The vertex of the parabola \( P_0 \) is at \( (0, 0) \). The centroid \( G \) of the triangle formed by \( A \), \( B \), and the vertex is:
   \[
   G = \left( \frac{0 + t + u}{3}, \frac{0 + mt^2 + mu^2}{3} \right)
   \]
   Using \( tu = -\frac{1}{4m^2} \), we can express \( u \) in terms of \( t \):
   \[
   u = -\frac{1}{4m^2t}
   \]
   Substituting \( u \) into the centroid coordinates:
   \[
   G = \left( \frac{t - \frac{1}{4m^2t}}{3}, \frac{mt^2 + m\left(-\frac{1}{4m^2t}\right)^2}{3} \right)
   \]
   Simplifying the coordinates:
   \[
   G = \left( \frac{t - \frac{1}{4m^2t}}{3}, \frac{mt^2 + \frac{m}{16m^4t^2}}{3} \right)
   \]
   \[
   G = \left( \frac{t - \frac{1}{4m^2t}}{3}, \frac{mt^2 + \frac{1}{16m^3t^2}}{3} \right)
   \]

5. **Locus of the Centroid:**
   To find the locus of \( G \), we eliminate \( t \). Let \( x = \frac{t - \frac{1}{4m^2t}}{3} \) and \( y = \frac{mt^2 + \frac{1}{16m^3t^2}}{3} \). Multiply both sides of the first equation by 3:
   \[
   3x = t - \frac{1}{4m^2t}
   \]
   Solving for \( t \):
   \[
   3xt = t^2 - \frac{1}{4m^2}
   \]
   \[
   t^2 - 3xt - \frac{1}{4m^2} = 0
   \]
   This is a quadratic equation in \( t \). Solving for \( t \):
   \[
   t = \frac{3x \pm \sqrt{(3x)^2 + \frac{1}{m^2}}}{2}
   \]
   Substituting \( t \) back into the expression for \( y \):
   \[
   y = \frac{m \left( \frac{3x \pm \sqrt{(3x)^2 + \frac{1}{m^2}}}{2} \right)^2 + \frac{1}{16m^3 \left( \frac{3x \pm \sqrt{(3x)^2 + \frac{1}{m^2}}}{2} \right)^2}}{3}
   \]
   Simplifying this expression will show that the locus of \( G \) is a parabola of the form \( y = 3mx^2 + \frac{1}{6m} \).

6. **Inductive Step:**
   Repeating the same process for \( P_1 \), we get another parabola \( P_2 \), and so on. The general form of the \( n \)-th parabola \( P_n \) can be derived using induction:
   \[
   P_n: y = 3^n mx^2 + \frac{1}{6m} \left( 1 + \frac{1}{3} + \frac{1}{9} + \cdots + \frac{1}{3^{n-1}} \right)
   \]
   The series \( 1 + \frac{1}{3} + \frac{1}{9} + \cdots + \frac{1}{3^{n-1}} \) is a geometric series with sum:
   \[
   S = \frac{1 - \left( \frac{1}{3} \right)^n}{1 - \frac{1}{3}} = \frac{3}{2} \left( 1 - \left( \frac{1}{3} \right)^n \right)
   \]
   Therefore, the equation of \( P_n \) is:
   \[
   y = 3^n mx^2 + \frac{1}{6m} \cdot \frac{3}{2} \left( 1 - \left( \frac{1}{3} \right)^n \right)
   \]
   Simplifying:
   \[
   y = 3^n mx^2 + \frac{1}{4m} \left( 1 - \left( \frac{1}{3} \right)^n \right)
   \]

The final answer is \( \boxed{ y = 3^n mx^2 + \frac{1}{4m} \left( 1 - \left( \frac{1}{3} \right)^n \right) } \).