Let $\alpha_a$ denote the greatest odd divisor of a natural number $a$, and let $S_b=\sum_{a=1}^b\frac{\alpha_a}a$ Prove that the sequence $S_b/b$ has a finite limit when $b\to\infty$, and find this limit.
1. **Define the greatest odd divisor**: Let $\alpha_a$ denote the greatest odd divisor of a natural number $a$. For example, if $a = 12$, then $\alpha_{12} = 3$ because the odd divisors of 12 are 1 and 3, and the greatest is 3.

2. **Define the sequence \( S_b \)**: We are given the sequence \( S_b = \sum_{a=1}^b \frac{\alpha_a}{a} \). We need to prove that the sequence \( \frac{S_b}{b} \) has a finite limit as \( b \to \infty \), and find this limit.

3. **Upper bound for \( \frac{S_b}{b} \)**: Notice that for each \( a \in \mathbb{N} \), we have \( \alpha_a \le a \). Therefore,
   \[
   \frac{\alpha_a}{a} \le 1 \implies S_b = \sum_{a=1}^b \frac{\alpha_a}{a} \le \sum_{a=1}^b 1 = b.
   \]
   This implies
   \[
   \frac{S_b}{b} \le 1.
   \]

4. **Characterize the set \( A(t) \)**: Fix an odd positive integer \( t \) and let \( A(t) \) denote the set of all positive integers \( k \) for which \( \alpha_k = t \). That is,
   \[
   A(t) := \{ k \in \mathbb{N} \mid \alpha_k = t \}.
   \]
   A positive integer \( g \) has \( \alpha_g = q \) for an odd positive integer \( q \) if and only if \( q \mid g \) and \( g/q \) is not divisible by any odd number other than 1. Thus,
   \[
   A(t) = \{ t \cdot 2^k \mid k \in \mathbb{N}_0 \}.
   \]

5. **Sum over \( A(t) \)**: For each odd \( t \in \mathbb{N} \), we have
   \[
   \sum_{j \in A(t)} \frac{\alpha_j}{j} = \sum_{k=0}^{\infty} \frac{t}{t \cdot 2^k} = \sum_{k=0}^{\infty} \frac{1}{2^k} = 2.
   \]

6. **Upper bound using \( A(t) \)**: Notice that
   \[
   \sum_{a=1}^{b} \frac{\alpha_a}{a} \le \sum_{\substack{1 \le t \le b \\ t \text{ is odd}}} \sum_{j \in A(t)} \frac{\alpha_j}{j}.
   \]
   Since the number of odd numbers in the interval \([1, b]\) is approximately \( \frac{b}{2} \), we have
   \[
   \sum_{a=1}^{b} \frac{\alpha_a}{a} \le 2 \cdot \frac{b}{2} = b.
   \]

7. **Asymptotic equivalence**: We have shown that
   \[
   \frac{S_b}{b} \le 1.
   \]
   To show that \( \lim_{b \to \infty} \frac{S_b}{b} = 1 \), we use the fact that the number of odd numbers in \([1, b]\) is asymptotically \( \frac{b}{2} \). Therefore, the sum \( S_b \) is asymptotically equivalent to \( b \), and thus
   \[
   \lim_{b \to \infty} \frac{S_b}{b} = 1.
   \]

The final answer is \( \boxed{ 1 } \).