Largest prime < n!! (double factorial of n).

Terms

a(0) =2a(1) =7a(2) =13a(3) =47a(4) =103a(5) =383a(6) =941a(7) =3833a(8) =10391a(9) =46073a(10) =135131a(11) =645097a(12) =2027023a(13) =10321919a(14) =34459423a(15) =185794463a(16) =654729073a(17) =3715891181a(18) =13749310511a(19) =81749606383a(20) =316234143163