cp's OEIS Frontend

A271116 Integers n such that round(3^n/12) is divisible by n.

%I A271116 #34 Oct 30 2020 15:48:09
%S A271116 1,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,91,
%T A271116 97,101,103,107,109,113,121,127,131,137,139,149,151,157,163,167,173,
%U A271116 179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307
%N A271116 Integers n such that round(3^n/12) is divisible by n.
%C A271116 In other words, numbers n such that A015518(n-1) is divisible by n.
%C A271116 This sequence generates prime numbers except 2 and 3.
%C A271116 The first few composite terms in this sequence are 91, 121, 671, 703, 949. Note that they are pseudoprimes to base 3.
%C A271116 Also, numbers k such that 3^(k-1) mod (4*k) = 1. In the first million terms only 908 terms are nonprimes. - _David A. Corneth_, Oct 02 2020
%H A271116 David A. Corneth, <a href="/A271116/b271116.txt">Table of n, a(n) for n = 1..10000</a>
%e A271116 5 is a term because round(3^5/12) = 20 is divisible by 5.
%e A271116 6 is not a term because round(3^6/12) = 61 which is not divisible by 6. - _David A. Corneth_, Oct 02 2020
%t A271116 Select[Range@ 308, Divisible[Round[3^#/12], #] &] (* _Michael De Vlieger_, Mar 31 2016 *)
%o A271116 (PARI) f(n) = round(3^n/12);
%o A271116 for(n=1, 1e3, if(f(n) % n == 0, print1(n, ", ")));
%o A271116 (PARI) is(n) = lift(Mod(3, 4*n)^(n-1))==1 \\ _David A. Corneth_, Oct 02 2020
%Y A271116 Cf. A005935, A015518.
%K A271116 nonn,easy
%O A271116 1,2
%A A271116 _Altug Alkan_, Mar 31 2016