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

3, 17, 13, 5, 53, 149, 1237, 1109, 853, 2389, 3413, 17749, 128341, 70997, 251221, 415061, 218453, 2708821, 27088213, 29709653, 3495253, 85284181, 13981013, 39146837, 794121557, 1498764629, 492131669, 626349397, 13779686741

Offset

1,1

Links

Giovanni Resta, Table of n, a(n) for n = 1..1000

Formula

a(n) = A000040(Min{x; A007814(1 + 3*A000040(x)) = n}).

Example

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.

Maple

f:= proc(n)
   local m, t, p;
   t:= 2^n;
   for m from 1 + 4*(n mod 2) by 6 do
     p:= (t*m-1)/3;
     if isprime(p) then return p fi
   od
end proc:
map(f, [$1..100]); # Robert Israel, Nov 18 2017

Mathematica

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]]]];
Array[a, 100] (* Jean-François Alcover, Aug 28 2020, after Robert Israel *)

Crossrefs

Cf. A087272, A087273, A087274, A007814, A087230, A087963.

Sequence in context: A357373 A273702 A273710 * A372798 A174182 A120448

Adjacent sequences: A087961 A087962 A087963 * A087965 A087966 A087967

Keyword

nonn

Author

Labos Elemer, Sep 18 2003

Extensions

More terms from Ray Chandler, Sep 21 2003

Status

approved