A036798 - OEIS

A036798

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

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

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OFFSET

1,1

COMMENTS

These numbers m appear to satisfy cototient(m) > totient(m) or 2*phi(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(m) < m, 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