for(int i = 0; i < _size - _k; i+=_k)
    for(int j = 0; j < _size -_k; j+=_k)
        a += 1.0/2.0* (powf(_box[i+_k][j] - _box[i][j],2) + powf(_box[i][j+_k] - _box[i][j],2.0)) - _densitybox[i][j] * _box[i][j] * _k * _k * _dx *_dy;