A184968 Smallest k such that phi(phi(k)) = 2^n.

5, 11, 17, 41, 85, 137, 257, 641, 1285, 2329, 4369, 10537, 17477, 35209, 65537, 163841, 297109, 557057, 1114129, 2687017, 4491589, 8978569, 16843009, 42009217, 71304257, 143163649, 286331153, 690563369, 1145390149, 2281701377, 4295098369, 10737647617

Offset

1,1

Comments

Phi is Euler's function.

Links

Robert Israel, Table of n, a(n) for n = 1..1000

Example

a(5) = 85 because phi(85) = 64, phi(64) = 2^5.

Maple

with(numtheory):for n from 1 to 22 do: id:=0:for k from 1 to 10000000 while(id=0)
  do: if phi(phi(k)) =2^n then id:=1:print(k):else fi:od:od:
# Alternative:
f:= proc(n) local S, s, r;
  uses numtheory;
  S:= sort(convert(invphi(2^n), list));
  r:= infinity;
  for s in S while s < r do
    r:= min(r, min(invphi(s)))
  od;
  r
end proc:
map(f, [$1..50]); # Robert Israel, Mar 22 2017

Crossrefs

Cf. A000010.

Sequence in context: A088903 A094205 A136091 * A341357 A022004 A339503

Adjacent sequences: A184965 A184966 A184967 * A184969 A184970 A184971

Keyword

nonn

Author

Michel Lagneau, Mar 27 2011

Extensions

a(23)-a(32) from Donovan Johnson, Jul 28 2011

Status

approved