cp's OEIS Frontend

A087964 a(n) is the least prime p such that exponent of highest power of 2 dividing 3p+1 equals n.

%I A087964 #23 Aug 28 2020 02:37:14
%S A087964 3,17,13,5,53,149,1237,1109,853,2389,3413,17749,128341,70997,251221,
%T A087964 415061,218453,2708821,27088213,29709653,3495253,85284181,13981013,
%U A087964 39146837,794121557,1498764629,492131669,626349397,13779686741
%N A087964 a(n) is the least prime p such that exponent of highest power of 2 dividing 3p+1 equals n.
%H A087964 Giovanni Resta, <a href="/A087964/b087964.txt">Table of n, a(n) for n = 1..1000</a>
%F A087964 a(n) = A000040(Min{x; A007814(1 + 3*A000040(x)) = n}).
%e A087964 p = 218453 is the first prime so that 3*p+1 = 655360 = (2^18)*5 has 18 as exponent of 2 in 3p+1, thus a(18) = 218453.
%p A087964 f:= proc(n)
%p A087964    local m,t,p;
%p A087964    t:= 2^n;
%p A087964    for m from 1 + 4*(n mod 2) by 6 do
%p A087964      p:= (t*m-1)/3;
%p A087964      if isprime(p) then return p fi
%p A087964    od
%p A087964 end proc:
%p A087964 map(f, [$1..100]); # _Robert Israel_, Nov 18 2017
%t A087964 a[n_] := Module[{m, t = 2^n, p}, For[m = 1 + 4 Mod[n, 2], True, m += 6, p = (t m - 1)/3; If[PrimeQ[p], Return[p]]]];
%t A087964 Array[a, 100] (* _Jean-François Alcover_, Aug 28 2020, after _Robert Israel_ *)
%Y A087964 Cf. A087272, A087273, A087274, A007814, A087230, A087963.
%K A087964 nonn
%O A087964 1,1
%A A087964 _Labos Elemer_, Sep 18 2003
%E A087964 More terms from _Ray Chandler_, Sep 21 2003