a(n) = (2n-1)!!/lcm{1,3,5,...,2n-1}.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =3a(6) =3a(7) =45a(8) =45a(9) =45a(10) =945a(11) =945a(12) =4725a(13) =42525a(14) =42525a(15) =42525a(16) =1403325a(17) =49116375a(18) =49116375a(19) =1915538625a(20) =1915538625a(21) =1915538625a(22) =86199238125a(23) =86199238125a(24) =603394666875