%I #31 Mar 30 2026 23:59:08

%S 15,51,85,91,255,435,451,561,595,771,1105,1261,1285,1351,1695,2091,

%T 2431,2465,3655,3855,4369,4795,5083,5151,5383,6601,6643,6735,7051,

%U 8245,8481,8695,8911,8995,9061,9605,10585,11155,13107,15051,15211,16405,16705,17733,18721,19669,20451,21845,22359,23001,26335,28645

%N Odd composite numbers k such that A053575(k) [the odd part of phi] divides k-1.

%C No common terms with A016105. See A339870 for the reason. - _Antti Karttunen_, Dec 26 2020

%H Antti Karttunen, <a href="/A339880/b339880.txt">Table of n, a(n) for n = 1..1237</a>

%t q[k_] := OddQ[k] && CompositeQ[k] && Divisible[k-1, (# / 2^IntegerExponent[#, 2])& @ EulerPhi[k]]; Select[Range[30000], q] (* _Amiram Eldar_, Mar 30 2026 *)

%o (PARI)

%o A000265(n) = (n>>valuation(n, 2));

%o isA339880(n) = (bitand(n,1)&&(n>1)&&!isprime(n)&&!((n-1)%A000265(eulerphi(n))));

%Y Cf. A000010, A000265, A016105, A053575.

%Y Subsequence of A005117 and of A339879, and of A340077.

%Y Cf. A339869, A339870 (subsequences).

%Y Cf. also A002997, A053576, A339817.

%K nonn,changed

%O 1,1

%A _Antti Karttunen_, Dec 24 2020