A341717 a(n) = smallest m >= n such that the decimal concatenation n||n+1||n+2||...||m is prime, or -1 if no such prime exists.

2, 3, 7, 5, 7, 7, 9, 187

Offset: 2

Author

N. J. A. Sloane, Feb 22 2021

Keywords

• nonn
• base
• more

Comments

These are the values of n+k from A341715. See that entry for more information.

See A084559 (the same for nonprime indices) for further information. - M. F. Hasler, Feb 22 2021

Suppose n > 3. If n == 2 (mod 3), then a(n) == 3 or 5 (mod 6). If n = 0 or 1 (mod 3), then a(n) == 1 (mod 6) (see A341716). - Chai Wah Wu, Feb 22 2021

Crossrefs

Cf. A084559, A140793, A341715, A341716.

Programs

Mathematica

Array[Block[{k = #, s = #}, While[! PrimeQ[s], k++; s = FromDigits[IntegerDigits[s]~Join~IntegerDigits[k]]]; k] &, 8, 2] (* Michael De Vlieger, Feb 22 2021 *)

PARI

A341717(n)=if(isprime(n), n, A084559(n)) \\ M. F. Hasler, Feb 22 2021

Formula

a(n) = n iff n is prime, otherwise a(n) = A084559(n). - M. F. Hasler, Feb 22 2021