Proposed edit to http://stackoverflow.com/a/1510186/849891

    Enumerable.Range(0, Math.Floor(Math.Sqrt(num)) ).Aggregate(
        Enumerable.Range(2, num).ToList(),
        (result, index) => {
            result.RemoveAll(i => i > result[index] && i % result[index] == 0);
            return result;
        }
    );