Find all positive integers $n$ such that there exists the polynomial with degree $n$ satisfying $f(x^2+1)=f(x)^2+1$.
1. **Assertion and Initial Setup:**
   Let \( P(x) \) be the assertion \( f(x^2 + 1) = f(x)^2 + 1 \). We need to find all positive integers \( n \) such that there exists a polynomial \( f(x) \) of degree \( n \) satisfying this functional equation.

2. **Odd Polynomial Case:**
   Suppose \( f(x) \) is an odd polynomial. Then we can write \( f(x) = xg(x^2) \) for some polynomial \( g(x) \). Substituting this into the functional equation, we get:
   \[
   f(x^2 + 1) = (x^2 + 1)g((x^2 + 1)^2)
   \]
   and
   \[
   f(x)^2 + 1 = (xg(x^2))^2 + 1 = x^2g(x^2)^2 + 1.
   \]
   Equating these, we have:
   \[
   (x^2 + 1)g((x^2 + 1)^2) = x^2g(x^2)^2 + 1.
   \]
   Simplifying, we get:
   \[
   (x^2 + 1)g((x^2 + 1)^2) = x^2g(x^2)^2 + 1.
   \]
   Setting \( x = 0 \), we get:
   \[
   g(1) = 1.
   \]
   Setting \( x = 1 \), we get:
   \[
   g(4) = 1.
   \]
   Setting \( x = 2 \), we get:
   \[
   g(25) = 1.
   \]
   Since \( g(x) = 1 \) for infinitely many values of \( x \), we conclude that \( g(x) = 1 \) for all \( x \). Thus, \( f(x) = x \).

3. **Even Polynomial Case:**
   If \( f(x) \) is even, then \( f(x) = g(x^2) \) for some polynomial \( g(x) \). Substituting this into the functional equation, we get:
   \[
   f(x^2 + 1) = g((x^2 + 1)^2)
   \]
   and
   \[
   f(x)^2 + 1 = g(x^2)^2 + 1.
   \]
   Equating these, we have:
   \[
   g((x^2 + 1)^2) = g(x^2)^2 + 1.
   \]
   Let \( h(x) = g(x + 1) \). Then the equation becomes:
   \[
   h((x + 1)^2 + 1) = h(x + 1)^2 + 1.
   \]
   Simplifying, we get:
   \[
   h(x^2 + 1) = h(x)^2 + 1.
   \]
   This is the same form as the original equation, indicating that \( f(x) \) has degree \( 2k \) and there exists a function \( h(x) \) of degree \( k \) which is also a solution.

4. **Existence of Solutions for Degrees \( 2^p \):**
   Let \( g(x) = x^2 + 1 \). The equation becomes:
   \[
   f(g(x)) = g(f(x)).
   \]
   This implies that any \( f(x) = g^{[p]}(x) \) (composition of \( g \) with itself \( p \) times) is a solution. The degree of \( g^{[p]}(x) \) is \( 2^p \).

5. **Conclusion:**
   Therefore, the only possible degrees for \( f(x) \) are \( 2^p \) for \( p \in \mathbb{N} \cup \{0\} \).

The final answer is \( \boxed{ n \in \{2^k \mid k \in \mathbb{N} \cup \{0\}\} } \).