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Numbers k such that the string 88k is prime.
1

%I #17 Sep 08 2022 08:45:19

%S 1,3,7,19,21,31,37,39,49,61,63,67,87,93,117,129,169,177,211,223,237,

%T 241,259,261,289,301,321,327,337,339,379,397,411,423,427,463,469,471,

%U 493,499,513,523,547,589,591,607,609,643,651,657,661,663,667,681,721,729

%N Numbers k such that the string 88k is prime.

%H Vincenzo Librandi, <a href="/A109991/b109991.txt">Table of n, a(n) for n = 1..2500</a>

%e If k=1 then 88k = 881, which is prime.

%e If k=61 then 88k = 8861, which is prime.

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

%p while not isprime(parse(""||88||k)) do od; a(n):=k

%p end:

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

%t Select[Range[900], PrimeQ[88 10^IntegerLength[#] + #] &] (* _Vincenzo Librandi_, Jul 30 2014 *)

%o (Magma) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [8, 8])) ]; // _Vincenzo Librandi_, Jul 30 2014

%K nonn,base

%O 1,2

%A _Parthasarathy Nambi_, Sep 01 2005

%E More terms from _Vincenzo Librandi_, Mar 27 2010