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A036798 Odd n such that there exists an even number k < n with phi(k) = phi(n). 10

%I

%S 105,165,195,315,495,525,585,735,825,945,975,1155,1365,1485,1575,1755,

%T 1785,1815,1995,2145,2205,2415,2475,2535,2625,2805,2835,2925,3003,

%U 3045,3135,3255,3315,3465,3675,3705,3795,3885,3927,4095,4125,4305,4389,4455

%N Odd n such that there exists an even number k < n with phi(k) = phi(n).

%C These numbers m appear to satisfy cototient[m] > totient[m] or 2phi[m] < m; - they seem to be the missing terms mentioned in A067800. - _Labos Elemer_, May 08 2003

%C All elements in this sequence must have 2*phi(n) < n, but not the reverse. See A118700. - _Franklin T. Adams-Watters_, May 21 2006

%H Robert Israel, <a href="/A036798/b036798.txt">Table of n, a(n) for n = 1..10000</a>

%p N:= 10^4: # to get all terms <= N

%p PhiE:= map(numtheory:-phi, [seq(i,i=2..N,2)]):

%p A:= NULL:

%p for n from 1 to N by 2 do

%p t:= numtheory:-phi(n);

%p if 2*t < n and member(t, PhiE[1..(n-1)/2]) then A:= A,n fi;

%p od:

%p A; # _Robert Israel_, Jan 06 2017

%t Select[ Range[1, 4483, 2], Mod[ #, EulerPhi[ # ]] != # - EulerPhi[ # ] &] (* _Robert G. Wilson v_, Jan 10 2004 *)

%Y Cf. A000010, A067800, A083254.

%Y Cf. A091495 (Odd, squarefree n such that n/phi(n) > 2).

%Y Cf A118700, A119434.

%K nonn

%O 1,1

%A _David W. Wilson_

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Last modified May 21 17:33 EDT 2019. Contains 323444 sequences. (Running on oeis4.)