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 A193910 #45 Nov 11 2019 21:53:23
%S A193910 2,6,11,15,20,24,29,33,38,42,47,51,56,60,65,69,74,78,83,87,92,96,101,
%T A193910 105,110,114,119,123,128,132,137,141,146,150,155,159,164,168,173,177,
%U A193910 182,186,191,195,200,204,209,213,218,222,227,231,236,240,245,249
%N A193910 Leap centuries in the revised Julian calendar.
%C A193910 Terms divided by 100, e.g., 29 indicates year 2900, which is a revised Julian leap year, but not a Gregorian leap year. Values below 20 are "proleptic" (only based on the formula).
%H A193910 M. Milankovitch, <a href="http://articles.adsabs.harvard.edu/full/seri/AN.../0220/0000203.000.html">Das Ende des julianischen Kalenders und der neue Kalender der orientalischen Kirchen, Astronomische Nachrichten, volume 220 (1924), 379-384.</a>
%H A193910 Claus Tøndering, Frequently Asked Questions about Calendars, <a href="http://www.tondering.dk/claus/cal/node3.html#SECTION00323000000000000000">Don't the Greeks do it differently?</a>
%H A193910 Wikipedia, <a href="http://en.wikipedia.org/wiki/Revised_Julian_calendar">Revised Julian calendar</a>
%H A193910 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A193910 a(n) = a(n-2) + 9. - _Charles R Greathouse IV_, Aug 09 2011
%F A193910 a(n) = 2 or 6 (mod 9).
%F A193910 For all positive integers n, a(n) = (1/4)*(18*n-17*(-1)^n-11), which implies a(2*n-1) = 9*n-3 and a(2*n) = 9*n-7. - _Farideh Firoozbakht_, Oct 08 2014
%F A193910 G.f.: x*(2 + 4*x + 3*x^2)/((1 + x)*(1 - x)^2). - _Philippe Deléham_, Nov 30 2016
%e A193910 20 mod 9 is 2; 2000 was a leap year in the revised Julian calendar.
%e A193910 24 mod 9 is 6; 2400 and 2000 also happen to be Gregorian leap years.
%e A193910 28 is the first integer greater than 16 only contained in A008586.
%e A193910 29 is the first integer greater than 16 not contained in A008586.
%t A193910 Table[1/4 (18 m - (-1)^m - 11), {m, 56}] (* _Farideh Firoozbakht_, Oct 08 2014 *)
%o A193910 (Rexx) do C = 0 to 250; J = C // 9; if J = 2 | J = 6 then say C; end C
%o A193910 (PARI) a(n)=(9*n-5)\2 \\ _Charles R Greathouse IV_, Aug 23 2011
%Y A193910 A008586 enumerates "Gregorian leap centuries" (N mod 4 = 0).
%Y A193910 A193879 enumerates all differences from A008586.
%Y A193910 Cf. A274406: numbers congruent to {0, 8} mod 9; A301451: numbers congruent to {1, 7} mod 9. This sequence lists the numbers congruent to {2, 6} mod 9.
%K A193910 nonn,easy
%O A193910 1,1
%A A193910 _Frank Ellermann_, Aug 09 2011