Code Words Parity A713Ab
1. **Problem Statement:**
Find the code words generated by the parity check matrix $$H=\begin{pmatrix}1 & 1 & 0 \\ 0 & 1 & 1 \\ 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}$$ when the encoding function is $$R : B^3 \to B^5$$.
2. **Understanding the Problem:**
The parity check matrix $$H$$ is a $$5 \times 3$$ matrix. The encoding function $$R$$ maps 3-bit vectors from $$B^3$$ to 5-bit code words in $$B^5$$.
3. **Key Concept:**
Code words $$\mathbf{c}$$ satisfy $$H \mathbf{c} = \mathbf{0}$$ over $$B$$ (binary field). Since $$H$$ is $$5 \times 3$$, the code words are in the image of $$R$$, which is a subspace of $$B^5$$.
4. **Step to find code words:**
Since $$R$$ maps $$B^3$$ to $$B^5$$, and $$H$$ is $$5 \times 3$$, the code words are generated by the null space of $$H^T$$ (transpose of $$H$$), i.e., $$\mathbf{c} = R(\mathbf{m})$$ for $$\mathbf{m} \in B^3$$.
5. **Calculate $$H^T$$:**
$$H^T = \begin{pmatrix}1 & 0 & 1 & 0 & 0 \\ 1 & 1 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 & 1 \end{pmatrix}$$
6. **Find generator matrix $$G$$ such that $$G H^T = 0$$:**
Since $$H$$ is parity check matrix, $$G$$ is a $$3 \times 5$$ matrix whose rows form a basis for the null space of $$H^T$$.
7. **Express $$G$$ in systematic form:**
We want $$G = [I_3 | P]$$ where $$I_3$$ is $$3 \times 3$$ identity and $$P$$ is $$3 \times 2$$ matrix.
8. **From $$H$$, identify $$P$$:**
Rewrite $$H$$ as $$H = \begin{pmatrix}P^T & I_2\end{pmatrix}$$ where $$I_2$$ is $$2 \times 2$$ identity.
Given $$H$$ is $$5 \times 3$$, this is not standard form, so we find $$G$$ by solving $$H G^T = 0$$.
9. **Enumerate all $$\mathbf{m} \in B^3$$:**
$$B^3 = \{(0,0,0), (0,0,1), (0,1,0), (0,1,1), (1,0,0), (1,0,1), (1,1,0), (1,1,1)\}$$
10. **Calculate code words $$\mathbf{c} = R(\mathbf{m})$$:**
Since $$R$$ is encoding function, and $$H$$ is parity check matrix, code words satisfy $$H \mathbf{c} = 0$$.
11. **Find all $$\mathbf{c}$$ such that $$H \mathbf{c} = 0$$:**
Let $$\mathbf{c} = (c_1, c_2, c_3, c_4, c_5)^T$$.
From $$H \mathbf{c} = 0$$:
$$\begin{cases}
1 c_1 + 1 c_2 + 0 c_3 = 0 \\ 0 c_1 + 1 c_2 + 1 c_3 = 0 \\ 1 c_1 + 0 c_2 + 0 c_3 = 0 \\ 0 c_1 + 1 c_2 + 0 c_3 = 0 \\ 0 c_1 + 0 c_2 + 1 c_3 = 0
\end{cases}$$
Simplify:
- From row 3: $$c_1 = 0$$
- From row 4: $$c_2 = 0$$
- From row 5: $$c_3 = 0$$
Then from row 1: $$c_1 + c_2 = 0$$ is true.
From row 2: $$c_2 + c_3 = 0$$ is true.
So $$c_1 = c_2 = c_3 = 0$$.
12. **Conclusion:**
The only code word is the zero vector $$\mathbf{c} = (0,0,0,0,0)^T$$.
**Final answer:**
The code words generated by $$H$$ under $$R$$ are only the zero vector $$\boxed{(0,0,0,0,0)}$$.
This means the code defined by $$H$$ and $$R$$ is trivial with only one code word.