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 A011763 #15 Oct 13 2022 15:28:24
%S A011763 365,365,365,366,365,365,365,366,365,365,365,366,365,365,365,366,365,
%T A011763 365,365,366,365,365,365,366,365,365,365,366,365,365,365,366,365,365,
%U A011763 365,366,365,365,365,366,365,365,365,366,365,365,365,366
%N A011763 Days in year in proleptic Gregorian calendar.
%C A011763 This sequence is periodic with period 400. The exceptions to the apparent period 4 are a(100) = a(200) = a(300) = 365. - _Franklin T. Adams-Watters_, Jul 10 2009
%H A011763 <a href="/index/Ca#calendar">Index entries for sequences related to calendars</a>
%H A011763 <a href="/index/Rec#order_400">Index entries for linear recurrences with constant coefficients</a>, order 400.
%t A011763 (* first do *) Needs["Calendar`"] (* then *) Table[ DaysBetween[{y, 1, 1}, {y + 1, 1, 1}], {y, 48}] (* _Robert G. Wilson v_, Apr 18 2010 *)
%o A011763 (PARI) a(n)=if(n%4, 365, if(n%100, 366, n%400, 366, 365)) \\ _Charles R Greathouse IV_, Oct 13 2022
%K A011763 nonn,easy
%O A011763 1,1
%A A011763 Juan-Carlos Lerman (jclerm(AT)aztec.asu.edu)