Semiprimes (A001358) whose digit reversal is a triangular number.

Terms

a(0) =6a(1) =10a(2) =51a(3) =55a(4) =82a(5) =87a(6) =309a(7) =501a(8) =649a(9) =694a(10) =1198a(11) =1207a(12) =1473a(13) =1765a(14) =1803a(15) =1837a(16) =1959a(17) =1981a(18) =3043a(19) =3057a(20) =3561a(21) =3574a(22) =5158a(23) =5221a(24) =5721a(25) =6139a(26) =6193a(27) =6267a(28) =6535a(29) =6711