The concatenation of n with n-1 and n with n+1 both yield primes (twin primes).

Terms

a(0) =42a(1) =78a(2) =102a(3) =108a(4) =180a(5) =192a(6) =270a(7) =300a(8) =312a(9) =330a(10) =342a(11) =390a(12) =420a(13) =522a(14) =540a(15) =612a(16) =660a(17) =822a(18) =840a(19) =882a(20) =1002a(21) =1140a(22) =1230a(23) =1272a(24) =1482a(25) =1542a(26) =1632a(27) =1770a(28) =2100a(29) =2190