a(n) = smallest positive number k with all digits equal such that the concatenation k||n||k is prime, or -1 if no such k exists.
A338366
a(n) = smallest positive number k with all digits equal such that the concatenation k||n||k is prime, or -1 if no such k exists.
Terms
- a(0) =1a(1) =3a(2) =7a(3) =1a(4) =11a(5) =1a(6) =7777a(7) =3a(8) =1a(9) =1a(10) =9a(11) =-1a(12) =7a(13) =33a(14) =99a(15) =1a(16) =3a(17) =1a(18) =1a(19) =9a(20) =1a(21) =11a(22) =-1a(23) =1a(24) =7a(25) =3a(26) =7777777777a(27) =1111a(28) =111a(29) =1
External references
- oeis: A338366