A cube with edge length $2n+ 1$ is dissected into small cubes of size  $1\times 1\times 1$ and bars of size $2\times 2\times 1$. Find the least possible number of cubes in such a dissection.

1. **Understanding the Problem:**
   We are given a cube with edge length \(2n + 1\). This cube is to be dissected into smaller cubes of size \(1 \times 1 \times 1\) and bars of size \(2 \times 2 \times 1\). We need to find the least possible number of \(1 \times 1 \times 1\) cubes in such a dissection.

2. **Initial Observation:**
   The total volume of the cube is \((2n + 1)^3\). Each \(1 \times 1 \times 1\) cube has a volume of 1, and each \(2 \times 2 \times 1\) bar has a volume of 4. We need to cover the entire volume using these pieces.

3. **Minimum Number of Small Cubes:**
   We need to show that at least \(2n + 1\) small cubes are required. Consider a plane of size \(1 \times (2n + 1) \times (2n + 1)\) within the cube. This plane cannot be completely covered by the \(2 \times 2 \times 1\) bars alone because each bar covers an even number of unit cubes (4), and the plane has an odd number of unit cubes \((2n + 1)^2\). Therefore, at least one \(1 \times 1 \times 1\) cube is required for each such plane.

4. **Inductive Construction:**
   We will use induction to show that the cube can be filled with exactly \(2n + 1\) small cubes.

   - **Base Case:**
     For \(n = 0\), the cube has edge length \(2 \cdot 0 + 1 = 1\). It is already a \(1 \times 1 \times 1\) cube, so we need exactly one small cube. This matches \(2 \cdot 0 + 1 = 1\).

   - **Inductive Step:**
     Assume that a cube of edge length \(2k + 1\) can be filled with \(2k + 1\) small cubes and the rest with bars. We need to show that a cube of edge length \(2(k + 1) + 1 = 2k + 3\) can be filled with \(2(k + 1) + 1 = 2k + 3\) small cubes.

     Consider the \(2k + 1\) cube in the center of the \(2k + 3\) cube. By the inductive hypothesis, this can be filled with \(2k + 1\) small cubes. The remaining volume forms a shell around this central cube. This shell has a thickness of 1 unit and can be divided into six faces, each of size \(2k + 3 \times 2k + 3 \times 1\).

     For each face, we can place \(2k + 1\) bars of size \(2 \times 2 \times 1\) such that only two opposing corners are left empty. This leaves two small cubes per face, totaling \(2 \times 6 = 12\) small cubes. However, these 12 small cubes are shared among the faces, and the total number of unique small cubes required is \(2k + 3\).

5. **Conclusion:**
   By induction, the least number of \(1 \times 1 \times 1\) cubes required to fill a cube of edge length \(2n + 1\) is \(2n + 1\).

The final answer is \(\boxed{2n + 1}\).