A120828 Numbers k such that the concatenation of n successive descending digits (1,0,9,8,7,...) starting with 1 is prime.

3, 5, 35, 139, 153, 253, 1053, 2015, 3703, 6933, 8173, 11959

Offset: 1

Author

Robert G. Wilson v, Jul 05 2006

Keywords

• nonn
• base
• hard
• more

Comments

Digits are in descending order beginning with 1 and after 0 comes 9.

Terms must end in 3, 5 or 9. - Michael S. Branicky, May 11 2023

a(13) > 10^5. - Michael S. Branicky, Apr 18 2025

Examples

			5 is a term since 10987 is a prime.
		

Crossrefs

Cf. A120804, A120819, A120829.

Programs

Maple

filter:= proc(n) local i; isprime( add(10^(n-1-i)*(1-i mod 10), i=0..n-1)) end proc:
select(filter, [$1..4000]); # Robert Israel, Mar 08 2023

Mathematica

fQ[n_] := PrimeQ@ FromDigits@ Mod[2-Range@n, 10]; lst = {}; Do[ If[fQ@n, AppendTo[lst, n]; Print@n], {n, 6000}]; lst

Extensions

Name corrected by Robert Israel, Mar 08 2023

a(10)-a(12) from Michael S. Branicky, May 11 2023