cp's OEIS Frontend

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

%I A143423 #11 Aug 28 2020 06:59:11
%S A143423 2,4,8,14,16,22,26,32,38,44,46,52,58,62,64,74,82,86,92,94,104,106,162,
%T A143423 116,118,122,128,134,142,146,158,164,166,172,178,188,194,202,206,212,
%U A143423 214,218,242,226,236,244,254,256,262,268,274,278,284,292,298,302,314
%N A143423 Least even number k such that phi(k) = n, where n runs through the values (A002202) taken by phi.
%C A143423 Such an even number always exists.
%H A143423 T. D. Noe, <a href="/A143423/b143423.txt">Table of n, a(n) for n=1..10000</a>
%H A143423 Maxim Rytin, <a href="http://library.wolfram.com/infocenter/MathSource/696/"> Finding the Inverse of Euler Totient Function </a> (1999).
%p A143423 f:= proc(m) local L;
%p A143423       L:= numtheory:-invphi(m);
%p A143423       if L = [] then NULL
%p A143423       else min(select(type,L,even))
%p A143423       fi
%p A143423 end proc:
%p A143423 map(f, [1,seq(2*k,k=1..1000)]); # _Robert Israel_, Oct 07 2015
%t A143423 f[m_] := Module[{L}, L = invphi[m]; If[L == {}, Nothing, Min[Select[L, EvenQ]]]];
%t A143423 f /@ Join [{1}, 2 Range[1000]] (* _Jean-François Alcover_, Aug 28 2020, using Maxim Rytin's invphi function *)
%Y A143423 Cf. A002181 (least k such that phi(k)=n), A006511 (largest k such that phi(k)=n).
%K A143423 nonn
%O A143423 1,1
%A A143423 _T. D. Noe_, Aug 14 2008