Number of matchings in the n-helm graph.

Terms

a(0) =2a(1) =3a(2) =10a(3) =29a(4) =82a(5) =227a(6) =618a(7) =1661a(8) =4418a(9) =11651a(10) =30506a(11) =79389a(12) =205522a(13) =529635a(14) =1359434a(15) =3476989a(16) =8865026a(17) =22538755a(18) =57157578a(19) =144615709a(20) =365127634a(21) =920110051a(22) =2314564522a(23) =5812911741a(24) =14576950082a(25) =36503608707a(26) =91294323178a(27) =228049363229a(28) =569017421650