1 def is_pow_2(n):

     2     bin_count = 0

     3     while n > 0:

     4         if n % 2 == 1:

     5             bin_count += 1

     6         if bin_count > 1:

     7             return False

     8         n /= 2

     9 

    10     return bin_count == 1

    11 

    12 def collatz_pow_2(n):

    13     if n == 1: return 4

    14     if n == 2: return 4

    15     collatz_pow_2 = []

    16     while n > 2:

    17         print n, 

    18         if is_pow_2(n):

    19             collatz_pow_2.append(n)

    20 

    21         if n % 2:

    22             n = n * 3 - 1

    23         else:

    24             n /= 2 

    25 

    26     return max(collatz_pow_2)

    27 

    28 import sys

    29 collatz_pow_2(int(sys.argv[1]))