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

Terms

a(0) =33a(1) =77a(2) =3131a(3) =1227a(4) =8191a(5) =1371a(6) =5287a(7) =2147a(8) =2351a(9) =6111a(10) =1627a(11) =1727a(12) =6851a(13) =5327a(14) =1087a(15) =1407a(16) =4451a(17) =2571a(18) =1991a(19) =2807a(20) =4711a(21) =3451a(22) =2891a(23) =4371a(24) =4451a(25) =4011a(26) =8571a(27) =5307a(28) =5207a(29) =5111