COMPOSE primes with natural numbers.

Terms

a(0) =2a(1) =7a(2) =23a(3) =75a(4) =242a(5) =772a(6) =2430a(7) =7529a(8) =22941a(9) =68801a(10) =203488a(11) =594994a(12) =1724242a(13) =4963041a(14) =14213111a(15) =40540661a(16) =115235882a(17) =326470662a(18) =921780312a(19) =2593408123a(20) =7269728291a(21) =20302519239a(22) =56493171410a(23) =156652350614a(24) =433009258768