The first 60 terms of the sequence enumerating unlabeled trees are [1, 1, 1, 2, 3, 6, 11, 23, 47, 106, 235, 551, 1301, 3159, 7741, 19320, 48629, 123867, 317955, 823065, 2144505, 5623756, 14828074, 39299897, 104636890, 279793450, 751065460, 2023443032, 5469566585, 14830871802, 40330829030, 109972410221, 300628862480, 823779631721, 2262366343746, 6226306037178, 17169677490714, 47436313524262, 131290543779126, 363990257783343, 1010748076717151, 2810986483493475, 7828986221515605, 21835027912963086, 60978390985918906, 170508699155987862, 477355090753926460, 1337946100045842285, 3754194185716399992, 10545233702911509534, 29650945107763261531, 83453838443384019701, 235105687101888719584, 662938933002209627441, 1870953015095482429652, 5284664207525664213829, 14939085337180746355566, 42263974955306727781419, 119658805094937105691820, 339028211512423891688777] This took , 0.802, seconds