Numbers k such that concat(k, sigma(k)) and concat(sigma(k), k) are both prime.

Terms

a(0) =1a(1) =49a(2) =961a(3) =1089a(4) =25281a(5) =52441a(6) =73441a(7) =76729a(8) =361201a(9) =375769a(10) =499849a(11) =1315609a(12) =1968409a(13) =2961841a(14) =3470769a(15) =4289041a(16) =4669921a(17) =4748041a(18) =5536609a(19) =7392961a(20) =7912969a(21) =9296401a(22) =9628609a(23) =10556001a(24) =13039321a(25) =13830961a(26) =14737921a(27) =14753281a(28) =17330569a(29) =17380561