Find the closed formula for each of the following sequences by relating them to a well known sequence. Assume the first term given is $a_1\text{.}$

1. $2, 5, 10, 17, 26, \ldots$
   $a_n =$
2. $0, 2, 5, 9, 14, 20, \ldots$
   $a_n =$
3. $8, 12, 17, 23, 30,\ldots$
   $a_n =$
4. $1, 5, 23, 119, 719,\ldots$
   $a_n =$