Primes starting and ending with 7.

Terms

a(0) =7a(1) =727a(2) =757a(3) =787a(4) =797a(5) =7027a(6) =7057a(7) =7127a(8) =7177a(9) =7187a(10) =7207a(11) =7237a(12) =7247a(13) =7297a(14) =7307a(15) =7417a(16) =7457a(17) =7477a(18) =7487a(19) =7507a(20) =7517a(21) =7537a(22) =7547a(23) =7577a(24) =7607a(25) =7687a(26) =7717a(27) =7727a(28) =7757a(29) =7817