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 A011765 #56 Dec 14 2023 05:51:25
%S A011765 0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,
%T A011765 0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,
%U A011765 0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1
%N A011765 Period 4: repeat [0, 0, 0, 1].
%C A011765 Arises in connection with leap years, years of U.S. Presidential elections, Olympic Games, etc.
%C A011765 Note that leap years define a sequence with period length 400, unlike A121262 which has period length 4. - _R. J. Mathar_, Dec 19 2008
%H A011765 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%H A011765 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,1).
%F A011765 G.f.: x^4/(1-x^4). - _Mohammad K. Azarian_, Dec 23 2008
%F A011765 a(n) = (1+(-1)^n)*(1+i^n)/4 with i=sqrt(-1). - _Bruno Berselli_, Mar 14 2011
%F A011765 a(n) = 1/4 - sin(Pi*(n-1)/2)/2 + (-1)^n/4. - _R. J. Mathar_, Oct 08 2011
%F A011765 The characteristic function of numbers that are multiples of 4. For the general case: the characteristic function of numbers that are multiples of m is a(n) = floor(n/m) - floor((n-1)/m), m,n > 0. - _Boris Putievskiy_, May 08 2013
%F A011765 a(n) = a(n-4) for n>4. - _Wesley Ivan Hurt_, Jul 10 2016
%p A011765 seq(op([0, 0, 0, 1]), n=1..50); # _Wesley Ivan Hurt_, Jul 10 2016
%t A011765 PadRight[{}, 120, {0,0,0,1}] (* or *) LinearRecurrence[{0,0,0,1}, {0,0,0,1}, 120] (* _Harvey P. Dale_, Aug 20 2012 *)
%o A011765 (PARI) a(n)=n%4==0 \\ _Charles R Greathouse IV_, Oct 07 2015
%o A011765 (Magma) &cat [[0, 0, 0, 1]^^30]; // _Wesley Ivan Hurt_, Jul 10 2016
%Y A011765 A121262 is another version.
%K A011765 nonn,easy
%O A011765 1,1
%A A011765 _N. J. A. Sloane_