A double factorial triangle.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =3a(5) =2a(6) =15a(7) =15a(8) =12a(9) =6a(10) =105a(11) =105a(12) =90a(13) =60a(14) =24a(15) =945a(16) =945a(17) =840a(18) =630a(19) =360a(20) =120a(21) =10395a(22) =10395a(23) =9450a(24) =7560a(25) =5040a(26) =2520a(27) =720a(28) =135135a(29) =135135