A032619 - OEIS

%I #15 Aug 31 2021 20:11:42

%S 3,5,17,27,35,39,45,47,63,65,83,93,99,113,123,147,149,173,185,203,209,

%T 213,219,227,239,249,255,263,267,285,287,303,309,317,327,329,333,363,

%U 365,399,413,419,423,435,447,459,465,467,473,489,497,509,515,519

%N Numbers k such that k concatenated with k+4 is a prime.

%H Robert Israel, <a href="/A032619/b032619.txt">Table of n, a(n) for n = 1..10000</a>

%p filter:= n -> isprime(n*(10^(1+ilog10(n+4))+1)+4):

%p select(filter, [seq(i,i=1..1000,2)]); # _Robert Israel_, Nov 14 2019

%t prkk4Q[k_]:=With[{c=k+4},PrimeQ[k*10^IntegerLength[c]+c]]; Select[ Range[ 600],prkk4Q] (* _Harvey P. Dale_, Aug 31 2021 *)

%Y Cf. A032609.

%K nonn,base

%O 1,1

%A _Patrick De Geest_, May 15 1998