I want to generate sub set of fibonacci series. But I am not getting enough idea to do this. Any help is appreciated.

public Task<IEnumerable<long>> GenerateSubset(int fromIndex, int toIndex)
    {
        int first =fromIndex;
        int second = fromIndex + 1;
        int third = 0;
        List<long> result = new List<long>();
        result.Add(first);
        result.Add(second);

        for (int i = fromIndex + 2; i <= toIndex - 1; i++)
        {
            third = first + second;
            result.Add(third);
            first = second;
            second = third;
        }
        yield return result;

    }