Bash integer expansion

How many ways bash can deal with integer expansion ? I only know 4 methods, I think there are more:
1. use let:

$ a=5
$ let b=a*6

1. use expr:

   $ a=5
   $ b=`expr $a \* 6`

2. use $[ ... ]:

   $ a=5
   $ b=$[ $a * 6 ]

3. use $(()):

   $ a=5
   $ b=$(( a * 6 ))

• in method 1 and 4, the variable a is "NAKED".