Alas, that version is only correct up to x = 25, because *Main> 1 / sqrt 5 * (((1 + sqrt 5) / 2) ^ 26 - ((1 - sqrt 5) / 2) ^ 26) 121392.99999999999 ha

Alas, that version is only correct up to x = 25, because *Main> 1 / sqrt 5 * (((1 + sqrt 5) / 2) ^ 26 - ((1 - sqrt 5) / 2) ^ 26) 121392.99999999999 happens to be just on the wrong side of the truncate.