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 A281898 #36 Jul 07 2025 15:39:47
%S A281898 0,1,2,4,5,8,10,13,17,20,25,29,34,40,45,50,58,65,80,85,97,100,125,130,
%T A281898 145,170,185,200,221,225,250,260,265,290,325,340,365,377,400,425,445,
%U A281898 450,485,493,520,530,545,580,625,650,680,685,730,754,765,785,800,841,845,890,900
%N A281898 Numbers k such that k - floor(sqrt(k))^2 and 2k - floor(sqrt(2k))^2 are both squares.
%C A281898 The sequence is infinite: it contains an infinite subsequence { A000129(2*k)^2 + 1, k>=0 }. - _Max Alekseyev_, Feb 01 2017
%C A281898 Also A000129(2k+1)^2 is a subsequence.
%C A281898 There are precisely six primes in this sequence: 2, 5, 13, 17, 29, and 97.
%H A281898 Charles R Greathouse IV, <a href="/A281898/b281898.txt">Table of n, a(n) for n = 1..10000</a>
%t A281898 Select[Range[0, 900], Times @@ Boole@ Map[IntegerQ@ Sqrt@ # &, # - Floor[Sqrt@ #]^2 &@ {#, 2 #}] == 1 &] (* _Michael De Vlieger_, Feb 02 2017 *)
%t A281898 Select[Range[0,1000],AllTrue[{Sqrt[#-Floor[Sqrt[#]]^2],Sqrt[2#-Floor[ Sqrt[ 2#]]^2]},IntegerQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* _Harvey P. Dale_, Apr 25 2020 *)
%o A281898 (PARI) is(n)=issquare(n-sqrtint(n)^2) && issquare(2*n-sqrtint(2*n)^2) \\ _Charles R Greathouse IV_, Feb 01 2017
%Y A281898 Cf. A000129, A076218 is a subsequence, A145016.
%K A281898 nonn
%O A281898 1,3
%A A281898 _Thomas Ordowski_, Feb 01 2017
%E A281898 More terms from _Altug Alkan_, Feb 01 2017