For Wednesday, Dec 6th, 2023:

  1. For the Hamming code example we used in the notes, i.e. with generating matrix $\begin{pmatrix} 1& 0 & 0 &0&1&1&0 \\ 0&1&0&0&1&0&1 \\ 0&0&1&0&0&1&1 \\ 0&0&0&1&1&1&1 \end{pmatrix}:
      1. Write out the parity check matrix and check that the first row of the generating matrix is a codeword.
      2. Suppose you receive the transmission 1000101.  Determine the syndrome.
      3. Determine the most likely codeword that was sent.
  2. In class, I listed out 8 elements of the example cyclic code given, which lives in the ring $R = \mathbb{F}_2[x]/(x^7-1)$.
      1. Write out all the 8 codewords we found in class, i.e. $g(x)x^k$ for various $k$.
      2.  I then pointed out that $g(x)(x^3 + x^2+1) = x^7-1$.  Explain why this means that $g(x)f(x) = g(x)f_1(x)$ for some $f_1(x)$ of degree $2$ or lower.
      3. How many elements of the ring $R$ of degree $\le 2$ are there?
      4. Explain why the above means the elements we already found are all the elements of this code.
  3. Consider the cyclic code generated by $g(x) = x^3 + 3x^2 + x + 6$ in the ring $R = \mathbb{F}_7[x]/(x^6-1)$.
      1. Find a generating matrix.
      2. What is the length of this code?
      3. What dimension is this code?
      4. What is the minimum distance of this code?
      5. Divide $x^6-1$ by $g(x)$ (polynomial long division!)
      6. We’ll continue this example in class.