A105042 - OEIS

%I #30 Feb 04 2024 01:19:27

%S 2,3,6,8,9,11,14,15,18,20,23,24,27,35,36,38,39,41,42,44,45,48,50,51,

%T 57,60,62,66,71,72,74,77,81,83,84,86,92,93,101,102,104,105,107,111,

%U 113,123,125,126,128,129,132,140,141,143,144,146,149,150,155,156,158,161,162

%N Numbers n such that 10n - 1 is prime.

%H Vincenzo Librandi, <a href="/A105042/b105042.txt">Table of n, a(n) for n = 1..1000</a>

%F a(k) = (A030433(k) + 1)/10. - _R. J. Mathar_, Jul 08 2009

%e If n = 2, then 10*n - 1 = 19 (prime).

%e If n = 18, then 10*n - 1 = 179 (prime).

%e If n = 36, then 10*n - 1 = 359 (prime).

%p a:= proc(n) if isprime(10*n-1) then n end if end proc: seq(a(n), n = 1 .. 200); # _Emeric Deutsch_, Aug 02 2009

%t Select[Range[200], PrimeQ[10# - 1] &] (* _Alonso del Arte_, Feb 09 2014 *)

%o (Magma) [n: n in [1..200]| IsPrime(10*n-1)]  // _Vincenzo Librandi_, Feb 10 2014

%o (PARI) is(n)=isprime(10*n-1) \\ _Charles R Greathouse IV_, Jun 12 2017

%K nonn,easy

%O 1,1

%A _Parthasarathy Nambi_, Apr 03 2005

%E Extended by _Emeric Deutsch_, Aug 02 2009