login
Numbers whose sum of proper divisors is odd.
7

%I #28 Jun 20 2021 03:11:10

%S 2,3,4,5,7,8,11,13,15,16,17,18,19,21,23,27,29,31,32,33,35,36,37,39,41,

%T 43,45,47,50,51,53,55,57,59,61,63,64,65,67,69,71,72,73,75,77,79,83,85,

%U 87,89,91,93,95,97,98,99,100,101,103,105,107,109,111,113,115,117,119

%N Numbers whose sum of proper divisors is odd.

%C Numbers which are twice squares, even squares or odd nonsquares.

%H Reinhard Zumkeller, <a href="/A053868/b053868.txt">Table of n, a(n) for n = 1..10000</a>

%o (Haskell)

%o a053868 n = a053868_list !! (n-1)

%o a053868_list = filter (odd . a001065) [1..]

%o -- _Reinhard Zumkeller_, Nov 01 2015, Sep 15 2011

%Y Cf. A000203, A001065, A028982, A053869.

%K nonn

%O 1,1

%A _Henry Bottomley_, Mar 29 2000

%E Improved name from _Omar E. Pol_, Aug 14 2009