a(n) = 1 + lcm(1, 2, ..., n) = 1 + A003418(n).

Terms

a(0) =2a(1) =2a(2) =3a(3) =7a(4) =13a(5) =61a(6) =61a(7) =421a(8) =841a(9) =2521a(10) =2521a(11) =27721a(12) =27721a(13) =360361a(14) =360361a(15) =360361a(16) =720721a(17) =12252241a(18) =12252241a(19) =232792561a(20) =232792561a(21) =232792561a(22) =232792561a(23) =5354228881a(24) =5354228881a(25) =26771144401a(26) =26771144401a(27) =80313433201a(28) =80313433201