a(n) = T(n,n-1), where T is the array in A026148.

Terms

a(0) =1a(1) =4a(2) =10a(3) =29a(4) =81a(5) =230a(6) =652a(7) =1854a(8) =5278a(9) =15048a(10) =42956a(11) =122772a(12) =351300a(13) =1006344a(14) =2885952a(15) =8285001a(16) =23809113a(17) =68490060a(18) =197211394a(19) =568385609a(20) =1639635613a(21) =4734036134a(22) =13679894484a(23) =39562904855a(24) =114507905551a(25) =331674931656a(26) =961408814434