a(0)=1, a(1)=5, a(n) = 20*a(n-2) - a(n-1).

Terms

a(0) =1a(1) =5a(2) =15a(3) =85a(4) =215a(5) =1485a(6) =2815a(7) =26885a(8) =29415a(9) =508285a(10) =80015a(11) =10085685a(12) =-8485385a(13) =210199085a(14) =-379906785a(15) =4583888485a(16) =-12182024185a(17) =103859793885a(18) =-347500277585