A109992 - OEIS

%I #20 Sep 08 2022 08:45:19

%S 1,7,23,29,31,41,49,67,73,103,109,119,131,133,137,139,149,173,181,191,

%T 223,233,241,251,257,259,277,289,317,347,349,367,371,377,391,397,401,

%U 409,431,439,469,487,497,523,527,529,551,559,563,571,577,581,607,611

%N Numbers n such that the string 99n is prime.

%H Vincenzo Librandi, <a href="/A109992/b109992.txt">Table of n, a(n) for n = 1..2500</a>

%e If n=103 then 99n = 99103 (prime).

%p a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))

%p       while not isprime(parse(""||99||k)) do od; a(n):=k

%p     end:

%p seq(a(n), n=1..100);  # _Alois P. Heinz_, Jul 30 2014

%t plQ[n_] := Module[{idn = IntegerDigits[n]}, PrimeQ[FromDigits[PadLeft[idn, Length[idn] + 2, 9]]]]; Select[Range[700], plQ] (* _Harvey P. Dale_, Jan 18 2011 *)

%t Select[Range[700], PrimeQ[99 10^IntegerLength[#] + #] &] (* _Vincenzo Librandi_, Jul 30 2014 *)

%o (Magma) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [9, 9])) ]; // _Vincenzo Librandi_, Jul 30 2014

%K nonn,base

%O 1,2

%A _Parthasarathy Nambi_, Sep 01 2005