In the post about the solving of the Euler problem 13, I was using an older version of F#, to fix the code, replace the 'N' with an 'I'. So for example, the bigNums collection looks like this: let bigNums = [37107287533902102798797998... 463769376774900097126481248... ... 535035342264725242508740540... And the 'AddNums' method is now: let rec AddNums n = if n < 0 then 0I else AddNums (n-1) + bigNums.[n] No biggies... (Apologies...