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A036798 Odd n such that there exists an even number k < n with phi(k) = phi(n). 10
105, 165, 195, 315, 495, 525, 585, 735, 825, 945, 975, 1155, 1365, 1485, 1575, 1755, 1785, 1815, 1995, 2145, 2205, 2415, 2475, 2535, 2625, 2805, 2835, 2925, 3003, 3045, 3135, 3255, 3315, 3465, 3675, 3705, 3795, 3885, 3927, 4095, 4125, 4305, 4389, 4455 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

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

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

MAPLE

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

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

A:= NULL:

for n from 1 to N by 2 do

t:= numtheory:-phi(n);

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

od:

A; # Robert Israel, Jan 06 2017

MATHEMATICA

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

CROSSREFS

Cf. A000010, A067800, A083254.

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

Cf A118700, A119434.

Sequence in context: A152940 A189936 A076762 * A119434 A091495 A256673

Adjacent sequences:  A036795 A036796 A036797 * A036799 A036800 A036801

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified April 20 17:54 EDT 2019. Contains 322310 sequences. (Running on oeis4.)