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A143423
Least even number k such that phi(k) = n, where n runs through the values (A002202) taken by phi.
1
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.
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
KEYWORD
nonn
AUTHOR
T. D. Noe, Aug 14 2008
STATUS
approved