1, 1, 1, 1, 2, 5, 15, 53, 222, 1078, 5994, 37622, 262776, 2018305, 16873364, 152233518, 1471613387, 15150569446, 165269824761, ... 

There is a lot of information in  [The On-Line Encyclopedia of Integer Sequences.][1]


  [1]: http://www.research.att.com/~njas/sequences/A006966