A140390 - OEIS

%I #16 May 16 2024 15:41:13

%S 1,22,64,85,106,148,169,190,232,253,274,295,316,358,400,442,484,505,

%T 526,568,589,610,652,694,715,736,778,799,820,841,862,904,925,946,988,

%U 1030,1072,1114,1135,1156,1177,1198,1219,1240,1261,1282,1324

%N Nonprimes of the form 21k+1.

%C The prime numbers 421=20*21+1 and 673=32*21+1, for example, are neither in here nor in A136069.

%H Vincenzo Librandi, <a href="/A140390/b140390.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[21 Range[0, 100] + 1, ! PrimeQ@# &] (* _Vincenzo Librandi_, Mar 22 2014 *)

%o (Magma) [a: n in [0..90] | not IsPrime(a) where a is 21*n+1]; // _Vincenzo Librandi_, Mar 22 2014

%Y Cf. A136069.

%K nonn,less

%O 1,2

%A _Juri-Stepan Gerasimov_, Jun 12 2008

%E Edited by _R. J. Mathar_, Jun 17 2008