cp's OEIS Frontend

A091113 Nonprimes of the form 4*k+1.

%I A091113 #26 Jul 02 2024 08:22:02
%S A091113 1,9,21,25,33,45,49,57,65,69,77,81,85,93,105,117,121,125,129,133,141,
%T A091113 145,153,161,165,169,177,185,189,201,205,209,213,217,221,225,237,245,
%U A091113 249,253,261,265,273,285,289,297,301,305,309,321,325,329,333,341,345
%N A091113 Nonprimes of the form 4*k+1.
%C A091113 A multiplicative semigroup: if m and n are in the sequence, then so is m*n. - _Antti Karttunen_, Jul 02 2024
%H A091113 Muniru A Asiru, <a href="/A091113/b091113.txt">Table of n, a(n) for n = 1..10000</a>
%p A091113 A091113 := proc(n)
%p A091113     option remember;
%p A091113     if n =1 then
%p A091113         1;
%p A091113     else
%p A091113         for a from procname(n-1)+4 by 4 do
%p A091113             if not isprime(a) then
%p A091113                 return a;
%p A091113             end if;
%p A091113         end do:
%p A091113     end if;
%p A091113 end proc:
%p A091113 seq(A091113(n),n=1..100) ; # _R. J. Mathar_, Aug 29 2018
%t A091113 Do[If[ !PrimeQ[n]&&Equal[Mod[n, 4], 1], Print[n]], {n, 1, 1000}]
%t A091113 Select[4*Range[0,100]+1,!PrimeQ[#]&] (* _Harvey P. Dale_, Oct 28 2017 *)
%o A091113 (GAP) Filtered(List([0..100],k->4*k+1),n->not IsPrime(n)); # _Muniru A Asiru_, Mar 10 2019
%o A091113 (Magma) [n: n in [1..350] | IsIntegral((n-1)/4) and not IsPrime(n)]; // _G. C. Greubel_, Mar 10 2019
%o A091113 (Sage) [n for n in (1..350) if ((n-1)/4).is_integer() and not is_prime(n)] # _G. C. Greubel_, Mar 10 2019
%o A091113 (PARI) isok(n) = !isprime(n) && !((n-1) % 4); \\ _Michel Marcus_, Mar 11 2019
%Y A091113 Cf. A014076, A091236, A373978 (characteristic function).
%Y A091113 Subsequence of A016813 (4*n+1).
%Y A091113 Cf. also A291745.
%K A091113 nonn
%O A091113 1,2
%A A091113 _Labos Elemer_, Feb 24 2004