>>>
>>>Off topic but since I think you enjoy obscure code I just wrote this (purely as a mockup):

static IEnumerable<int> GetSequence()
>>>        {
>>>            bool b = false;
>>>            int x1 = 0;
>>>            int x2 = 1;
>>>
>>>            while (true)
>>>            {
>>>                b=!b;
>>>                yield return b ? x1 += x2 : x2 += x1;
>>>            }
>>>        ]

>>
>>working my way through it with a pencil
>>
>>1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
>>
>>
>>If we number them as n0, n1, n2, n3, n4, ....
>>
>>then n[i] = n[i-1] + n[i-2]
>
>yep. Next in the series is the sum of the previous two items. It's called the Fibonacci series - but I actually got it wrong. It should be:
>0,1,1,2,3,5,8 etc


Take -1 as starting value for x1

  int x1 = -1;
  int x2 = 1;

As an alternative, you can do the calculation after the yield return