docs: protected DoubleMatrix getMatrix(int n)
      {
          Map<Integer, DoubleMatrix> degreeMap = matrixCache.get(this.getClass());
          if (degreeMap == null)
          {
              degreeMap = new HashMap<>();
              matrixCache.put(this.getClass(), degreeMap);
          }
          DoubleMatrix m = degreeMap.get(n);
          if (m == null)
          {
              m = createMatrix(n);
              m.decompose();
              degreeMap.put(n, m);
          }
          return m;
      }