%I #20 Dec 09 2019 07:26:49
%S 1,6,9,10,12,14,20,22,24,25,26,28,30,34,38,40,42,44,46,48,49,52,54,56,
%T 58,60,62,66,68,70,74,76,78,80,81,82,84,86,88,90,92,94,96,102,104,106,
%U 108,110,112,114,116,118,120,121,122,124,126,130,132,134,136,138,140
%N Sum of divisors of n less than n is even.
%C Numbers which if odd are squares and which if even are neither squares nor twice squares.
%C Numbers whose sum of proper divisors is even. - _Omar E. Pol_, Aug 14 2009
%H Reinhard Zumkeller, <a href="/A053869/b053869.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = 2n + 2*sqrt(n) + O(1). - _Charles R Greathouse IV_, Sep 27 2015
%o (Haskell)
%o a053869 n = a053869_list !! (n-1)
%o a053869_list = filter (even . a001065) [1..]
%o -- _Reinhard Zumkeller_, Nov 01 2015, Sep 15 2011
%o a053869_list = map (+ 1) $ findIndices even $ map a001065 [1..]
%o -- _Reinhard Zumkeller_, Sep 15 2011
%o (PARI) is(n)=if(n%2,issquare(n), !issquare(n) && !issquare(n/2)) \\ _Charles R Greathouse IV_, Sep 27 2015
%Y Cf. A000203, A001065, A028982, A053868.
%K nonn,easy
%O 1,2
%A _Henry Bottomley_, Mar 29 2000
%E Offset fixed by _Reinhard Zumkeller_, Sep 15 2011