Tetradic primes (primes in A006072).

Terms

a(0) =11a(1) =101a(2) =181a(3) =18181a(4) =1008001a(5) =1180811a(6) =1880881a(7) =1881881a(8) =100111001a(9) =100888001a(10) =108101801a(11) =110111011a(12) =111010111a(13) =111181111a(14) =118818811a(15) =180101081a(16) =181111181a(17) =181888181a(18) =188010881a(19) =188888881a(20) =10008180001a(21) =10081818001