Graham's number

graham.py

#!/usr/bin/python
# coding: utf-8
# Print Graham's number
# 
# Just a code for showing definition.
# It causes runtime error of maximum recursion.

def G(n):
    # G function of Graham's number
    if n == 0:
        return 4
    return uparrow(3,3,G(n-1))

def uparrow(a,b,n):
    # a↑^n b = a→b→n
    if b == 0:
        return 1
    if n == 1:
        return a**b
    return uparrow(a, uparrow(a, b-1, n), n-1)

# Print Graham's number
print(G(64))