True or false? (I don't know.) Every positive integer is the difference of two powers. Examples:
 $ 1 = 3^2 - 2^3 $   
 $ 2 = 3^3 - 5^2 $   
 $ 3  = 2^7-5^3 $  
 $ 4 = 2^3-2^2 = 5^3-11^2 $  
 $ 5 = 2^5 - 3^3 $  
 $ 6 = ? $  
 $ 7 = 2^5-5^2 $  
 $ 8 =? $  
 $ 9 = 5^2-2^4 $