Number of (binary) heaps of length n whose element set equals {1,2,3}.

Terms

a(0) =2a(1) =7a(2) =23a(3) =55a(4) =147a(5) =281a(6) =633a(7) =1241a(8) =2849a(9) =5187a(10) =11195a(11) =21369a(12) =47933a(13) =83821a(14) =174245a(15) =324571a(16) =712247a(17) =1263211a(18) =2660843a(19) =4999291a(20) =11056139a(21) =19236203a(22) =39793055a(23) =73882568a(24) =161646306a(25) =286178632a(26) =601791336a(27) =1129403320a(28) =2495184864a(29) =4326802772