Concatenation of first 3 digits and last 3 digits of n-th Mersenne prime A000668(n).

Terms

a(0) =33a(1) =77a(2) =3131a(3) =127127a(4) =819191a(5) =131071a(6) =524287a(7) =214647a(8) =230951a(9) =618111a(10) =162127a(11) =170727a(12) =686151a(13) =531127a(14) =104087a(15) =147007a(16) =446351a(17) =259071a(18) =190991a(19) =285607a(20) =478111a(21) =346551a(22) =281191a(23) =431471a(24) =448751a(25) =402511a(26) =854671a(27) =536207a(28) =521007a(29) =512311