I used mathematical analysis to find the two constants, from the iterative form: f(n) = f(n-1) + f(n-2) and initial conditions.
# Fibonacci #
from math import sqrt
c1 = (sqrt(5) - 1) / (2 * sqrt(5))
c2 = (sqrt(5) + 1) / (2 * sqrt(5))
def f(x):
fibo = c1 * ((1 - sqrt(5)) / 2)**x + c2 * ((1 + sqrt(5)) / 2)**x
return int(round(fibo))test.assert_equals(5, f(4), "")
print f(1)
print f(2)
print f(3)
print f(4)
print f(5)