Number[] a = new Number[p + 1]; // range from 1 to p

... some stuff to initialize a ...

Number[] ceps = new Number[Q + 1]; // range from 0 to Q

ceps[0] = ln(G);
for (int q = 1; q <= p; q++) {
  Number sum = a[q];
  for (k = 1; k <= q - 1; k++) {
    sum += (k - q) / q * a[k] * ceps[q - k];
  }
  ceps[q] = sum;
}

for (int q = p + 1; q <= Q; q++) {
  Number sum = 0;
  for (int k = 1; k <= p; k++) {
    sum += (k - q) / q * a[k] * ceps[q - k];
  }
  ceps[q] = sum;
}

(k - q) / q * a[k] * ceps[q - k]

((Number)(k - q))/((Number)q) * a[k] * ceps[q - k]

a[k] * ceps[q - k] * (k - q) / q