A143423 Least even number k such that phi(k) = n, where n runs through the values (A002202) taken by phi.

2, 4, 8, 14, 16, 22, 26, 32, 38, 44, 46, 52, 58, 62, 64, 74, 82, 86, 92, 94, 104, 106, 162, 116, 118, 122, 128, 134, 142, 146, 158, 164, 166, 172, 178, 188, 194, 202, 206, 212, 214, 218, 242, 226, 236, 244, 254, 256, 262, 268, 274, 278, 284, 292, 298, 302, 314

Offset

1,1

Comments

Such an even number always exists.

Links

T. D. Noe, Table of n, a(n) for n=1..10000

Maxim Rytin, Finding the Inverse of Euler Totient Function (1999).

Maple

f:= proc(m) local L;
      L:= numtheory:-invphi(m);
      if L = [] then NULL
      else min(select(type, L, even))
      fi
end proc:
map(f, [1, seq(2*k, k=1..1000)]); # Robert Israel, Oct 07 2015

Mathematica

f[m_] := Module[{L}, L = invphi[m]; If[L == {}, Nothing, Min[Select[L, EvenQ]]]];
f /@ Join [{1}, 2 Range[1000]] (* Jean-François Alcover, Aug 28 2020, using Maxim Rytin's invphi function *)

Crossrefs

Cf. A002181 (least k such that phi(k)=n), A006511 (largest k such that phi(k)=n).

Sequence in context: A086303 A209838 A121982 * A253142 A069049 A124853

Adjacent sequences: A143420 A143421 A143422 * A143424 A143425 A143426

Keyword

nonn

Author

T. D. Noe, Aug 14 2008

Status

approved