There are $n$ fleas on an infinite sheet of triangulated paper. Initially the fleas are in different small triangles, all of which are inside some equilateral triangle consisting of $n^2$ small triangles. Once a second each flea jumps from its original triangle to one of the three small triangles having a common vertex but no common side with it. For which natural numbers $n$ does there exist an initial configuration such that after a finite number of jumps all the $n$ fleas can meet in a single small triangle?
1. **Coloring the Triangles:**
   - Paint all triangles which point north (denoted as $\Delta$) red and the other triangles blue.
   - This creates a checkerboard pattern where each red triangle is surrounded by blue triangles and vice versa.

2. **Analyzing the Flea Jumps:**
   - Note that if a flea is on a red triangle, it will jump to a blue triangle, and if it is on a blue triangle, it will jump to a red triangle.
   - This means that fleas alternate between red and blue triangles with each jump.

3. **Initial Configuration:**
   - A necessary condition for all fleas to meet in a single triangle is that they must all start on triangles of the same color. This is because if they start on different colors, they will always be on different colors after each jump and can never meet in a single triangle.

4. **Feasibility of Meeting:**
   - Since any flea can go from any triangle to any other triangle (given enough jumps), we can pick any triangle and have all fleas go to it.
   - Once a flea reaches the target triangle, it can jump back and forth between the target triangle and one of its neighboring triangles to stay in the vicinity.

5. **Ensuring All Fleas Meet:**
   - Since the number of fleas is finite, and they can all be directed to the same triangle, eventually all the fleas will be on the same triangle after a finite number of jumps.

Conclusion:
- The necessary and sufficient condition for all fleas to meet in a single triangle is that they must all start on triangles of the same color.
- Given that the initial configuration is within an equilateral triangle consisting of $n^2$ small triangles, it is always possible to find such an initial configuration for any natural number $n$.

The final answer is $\boxed{n \text{ can be any natural number}}$