The concatenation pkp is the number obtained by placing prime p either side of R_k, the k-th repunit (1, k times); a(n) is the smallest k such that pkp is prime, where p=prime(n), or -1 if no such k exists.
A307873
The concatenation pkp is the number obtained by placing prime p either side of R_k, the k-th repunit (1, k times); a(n) is the smallest k such that pkp is prime, where p=prime(n), or -1 if no such k exists.
Terms
- a(0) =-1a(1) =1a(2) =-1a(3) =10905a(4) =15a(5) =2a(6) =1a(7) =2a(8) =3a(9) =1a(10) =3a(11) =173a(12) =1a(13) =14a(14) =1a(15) =43a(16) =1a(17) =5a(18) =11a(19) =1a(20) =2a(21) =3a(22) =3a(23) =1a(24) =2a(25) =-1a(26) =5a(27) =421a(28) =3a(29) =1
External references
- oeis: A307873