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Greatest number with totient 2n (or zero when no such number exists).
9

%I #7 Sep 14 2015 08:06:46

%S 6,12,18,30,22,42,0,60,54,66,46,90,0,58,62,120,0,126,0,150,98,138,94,

%T 210,0,106,162,174,118,198,0,240,134,0,142,270,0,0,158,330,166,294,0,

%U 276,0,282,0,420,0,250,206,318,214,378,242,348,0,354,0,462,0,0,254,510

%N Greatest number with totient 2n (or zero when no such number exists).

%C If a(n) = 0, n is a nontotient number - see (A005277)/2.

%H T. D. Noe, <a href="/A057826/b057826.txt">Table of n, a(n) for n=1..10000</a>

%t a = Table[0, {100}]; Do[ t = EulerPhi[n]/2; If[t < 101, a[[t]] = n], {n, 1, 10^3}]; a

%Y Bisection of A057635. Cf. A000010, A005277, A002181.

%K nonn

%O 1,1

%A _Robert G. Wilson v_, Nov 08 2000