Number of 4-ary search trees on n keys.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =4a(5) =10a(6) =20a(7) =47a(8) =128a(9) =340a(10) =868a(11) =2275a(12) =6188a(13) =16922a(14) =46112a(15) =126613a(16) =351568a(17) =981622a(18) =2747876a(19) =7723250a(20) =21811856a(21) =61828886a(22) =175752728a(23) =500984606a(24) =1432111244a(25) =4104175970a(26) =11787811340a(27) =33926475162a(28) =97837209036a(29) =282662156478