def pell(n):
"""
A little reading reveals that we can use continued fraction
convergents of the square root of n.
http://en.wikipedia.org/wiki/Pell%27s_equation
"""
cf = cf_sqrt(n)
if len(cf) == 1:
return None
for h, k in convergents(cf):
if h*h - n * k * k == 1:
return (h,k)
def p065():
n, d = itertools.islice(convergents(cf_e()), 99, 100).next()
return sum((int(c) for c in str(n)))
