This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A053869 #20 Dec 09 2019 07:26:49 %S A053869 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 A053869 58,60,62,66,68,70,74,76,78,80,81,82,84,86,88,90,92,94,96,102,104,106, %U A053869 108,110,112,114,116,118,120,121,122,124,126,130,132,134,136,138,140 %N A053869 Sum of divisors of n less than n is even. %C A053869 Numbers which if odd are squares and which if even are neither squares nor twice squares. %C A053869 Numbers whose sum of proper divisors is even. - _Omar E. Pol_, Aug 14 2009 %H A053869 Reinhard Zumkeller, <a href="/A053869/b053869.txt">Table of n, a(n) for n = 1..10000</a> %F A053869 a(n) = 2n + 2*sqrt(n) + O(1). - _Charles R Greathouse IV_, Sep 27 2015 %o A053869 (Haskell) %o A053869 a053869 n = a053869_list !! (n-1) %o A053869 a053869_list = filter (even . a001065) [1..] %o A053869 -- _Reinhard Zumkeller_, Nov 01 2015, Sep 15 2011 %o A053869 a053869_list = map (+ 1) $ findIndices even $ map a001065 [1..] %o A053869 -- _Reinhard Zumkeller_, Sep 15 2011 %o A053869 (PARI) is(n)=if(n%2,issquare(n), !issquare(n) && !issquare(n/2)) \\ _Charles R Greathouse IV_, Sep 27 2015 %Y A053869 Cf. A000203, A001065, A028982, A053868. %K A053869 nonn,easy %O A053869 1,2 %A A053869 _Henry Bottomley_, Mar 29 2000 %E A053869 Offset fixed by _Reinhard Zumkeller_, Sep 15 2011