>>
>>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