|
%I #24 Sep 08 2022 08:45:19
%S 7,19,21,27,31,57,63,69,73,81,91,103,109,117,127,147,163,171,201,207,
%T 213,217,219,229,243,249,259,291,313,331,333,337,339,343,351,373,381,
%U 399,411,439,441,457,469,487,501,511,529,541,547,579,589,603,609,619
%N Numbers n such that the string 55n is prime.
%H Vincenzo Librandi, <a href="/A109637/b109637.txt">Table of n, a(n) for n = 1..2000</a>
%e If n=7 then 55n = 557 which is a prime.
%e If n=81 then 55n = 5581 which is a prime.
%p a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))
%p while not isprime(parse(""||55||k)) do od; a(n):=k
%p end:
%p seq(a(n), n=1..100); # _Alois P. Heinz_, Jul 30 2014
%t Select[Range[1,1001,2],PrimeQ[FromDigits[Join[{5,5},IntegerDigits[#]]]]&] (* _Harvey P. Dale_, Dec 02 2010 *)
%t Select[Range[700], PrimeQ[55 10^IntegerLength[#] + #] &] (* _Vincenzo Librandi_, Jul 30 2014 *)
%o (Magma) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [5, 5])) ]; // _Vincenzo Librandi_, Jul 30 2014
%K nonn,base
%O 1,1
%A _Parthasarathy Nambi_, Aug 30 2005
%E More terms from _Vincenzo Librandi_, Mar 27 2010
|
