public void run() {
                            int idx1, idx2;
                            for (int k = firstIdx; k < lastIdx; k++) {
                                idx1 = 2 * k;
                                idx2 = offa + ((twon - idx1) % twon);
                                a[idx2] = a[offa + idx1];
                                a[idx2 + 1] = -a[offa + idx1 + 1];
                            }
                        }