a(n) = s(n+3)/6, where s is A024953.

Terms

a(0) =1a(1) =1a(2) =7a(3) =13a(4) =19a(5) =61a(6) =145a(7) =301a(8) =745a(9) =1729a(10) =4915a(11) =11737a(12) =28675a(13) =73945a(14) =200545a(15) =513265a(16) =1272385a(17) =3303745a(18) =8918911a(19) =23137501a(20) =59225587a(21) =155387413a(22) =420267097a(23) =1104554845a(24) =2876555737a(25) =7574538529a(26) =20668692859a(27) =54731279065a(28) =143892912259a(29) =383051857489