A145237 1/3 of the number of 3-colorings of an n X n array symmetric about main diagonal.
1, 4, 24, 216, 2952, 61488, 1957392, 95392296, 7125411888, 816458567400, 143601085430064, 38787148664334000, 16094818374887017440, 10263152688155204724288, 10059484565080507620249504
Offset: 1