A099070 Numbers k such that the concatenation of all nonprime natural numbers up to k with decreasing order is prime.

4, 5, 6, 7, 8, 9, 26, 1752, 1753

Offset: 1

Author

Farideh Firoozbakht, Nov 04 2004

Keywords

• base
• more
• nonn

Comments

If k is in the sequence and k+1 is prime then k+1 is also in the sequence. Next term is greater than 5450 and the prime corresponding to the next term has more than 18000 digits. Number of digits of primes corresponding to the nine known terms of the sequence are respectively 2,2,3,3,4,5,29,5010 & 5010.

a(10) > 20000. - Michael S. Branicky, Nov 25 2024

Examples

			9 is in the sequence because all nonprime natural numbers up to 9 are 1,4,6,8 & 9 and 98641 is prime.
		

Links

• Carlos Rivera, Puzzle 8. Primes by Listing, The Prime Puzzles & Problems connection.

Crossrefs

Cf. A046284, A099071.

Programs

Mathematica

Do[If[PrimeQ[(v={};Do[If[ !PrimeQ[n+1-j], v=Join[v, IntegerDigits [n+1-j]]], {j, n}];FromDigits[v])], Print[n]], {n, 5450}]