兒子推俺的例子

昨天兒子給俺發的email：

I have a problem for you:

Try to find the pattern in these sequences:

a) 21 is 9, 9 is 4, 4 is magic
13 is 8, 8 is 5, 5 is 4, 4 is magic
2,009 is 15, 15 is 7, 7 is 5, 5 is 4, 4 is magic

b) 1,11,21,1211,111221,312211,13112221,1113213211,31131211131221,13211311123113112211,...