A158019 - OEIS

%I #11 Dec 12 2024 20:51:11

%S 11,15,23,27,35,39,83,107,123,143,167,179,195,203,207,227,255,275,279,

%T 347,399,423,455,479,567,587,623,627,695,707,735,759,767,779,795,839,

%U 843,863,867,935,975,983,1007,1043,1095,1115,1155,1215,1227,1283,1323

%N Numbers such that (n-1)/2 and 10*n-1 are both prime.

%C Relating the two prime types:

%C (p-1)/2,(p-1)/4,(p-1)/6,(p-1)/8,(p-1)/10,(p-1)/12...

%C and

%C 2*p-1,4*p-1,6*p-1,8*p-1,10*p-1...

%C where both contain 11,23 subsequence.

%H Robert Israel, <a href="/A158019/b158019.txt">Table of n, a(n) for n = 1..10000</a>

%p filter:= proc(n) isprime((n-1)/2) and isprime(10*n-1) end proc:

%p select(filter, [seq(i,i=3..10000,4)]); # _Robert Israel_, Dec 12 2024

%t Flatten[Table[If[PrimeQ[(n - 1)/2] && PrimeQ[10*n - 1], n, {}], {n, 1, 10000}]]

%t Select[Range[1,1500,2], PrimeQ[(#-1)/2]&&PrimeQ[10#-1]&] (* _Harvey P. Dale_, Mar 10 2013 *)

%K nonn

%O 1,1

%A _Roger L. Bagula_ and _Gary W. Adamson_, Mar 11 2009, Mar 20 2009