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 A230998 #4 Nov 02 2013 23:38:37 %S A230998 1,2,3,4,8,9,10,13,14,15,16,19,20,21,24,25,26,27,30,31,32,36,37,38,41, %T A230998 42,43,44,47,48,49,52,53,54,55,58,59,60,64,65,66,69,70,71,72,75,76,77, %U A230998 80,81,82,83,86,87,88,92,93,94,97,98,99,100,103,104,105,109,110,111 %N A230998 Number of years after which it is not possible to have a date falling on the same day of the week, in the Gregorian calendar. %C A230998 In the Gregorian calendar, a non-century year is a leap year if and only if it is a multiple of 4 and a century year is a leap year if and only if it is a multiple of 400. %C A230998 Assuming this fact, this sequence is periodic with a period of 400. %C A230998 This is the complement of A230995. %H A230998 Time And Date, <a href="http://www.timeanddate.com/calendar/repeating.html">Repeating Calendar</a> %H A230998 Time And Date, <a href="http://www.timeanddate.com/calendar/gregorian-calendar.html">Gregorian Calendar</a> %o A230998 (PARI) for(i=0,400,j=0;for(y=0,400,if(((5*(y\4)+(y%4)-(y\100)+(y\400))%7)==((5*((y+i)\4)+((y+i)%4)-((y+i)\100)+((y+i)\400))%7),j=1));if(j==0,print1(i", "))) %Y A230998 Cf. A230995-A231014. %Y A230998 Cf. A231003 (Julian calendar). %K A230998 nonn,easy %O A230998 1,2 %A A230998 _Aswini Vaidyanathan_, Nov 02 2013