>>> def f(eps):
...     s = 0
...     n = 1
...     sl = 1/2 + 1/3
...     while sl >= eps:
...         s += sl
...         n += 1
...         sl = 1/(2**n) + 1/(3**n)
...     return s
... 
>>> f(0.001)
1.4980214723682874
>>> 
>>> f(0.000001)
1.4999980922211713
>>>