Natural numbers exponentiated twice.

Terms

a(0) =1a(1) =4a(2) =20a(3) =127a(4) =967a(5) =8549a(6) =85829a(7) =962308a(8) =11895252a(9) =160475855a(10) =2343491207a(11) =36795832297a(12) =617662302441