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A231236 Number of years after which it is either not possible to have a date falling on same day of the week, or the entire year can have the same calendar, in the Gregorian calendar.

%I A231236 #5 Nov 13 2013 15:18:42
%S A231236 0,1,2,3,4,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,
%T A231236 27,28,29,30,31,32,34,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,
%U A231236 52,53,54,55,56,57,58,59,60,62,64,65,66,67,68,69,70,71,72,73,74,75
%N A231236 Number of years after which it is either not possible to have a date falling on same day of the week, or the entire year can have the same calendar, in the Gregorian calendar.
%C A231236 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 A231236 Assuming this fact, this sequence is periodic with a period of 400.
%C A231236 This is the complement of A230997.
%H A231236 Time And Date, <a href="http://www.timeanddate.com/calendar/repeating.html">Repeating Calendar</a>
%H A231236 Time And Date, <a href="http://www.timeanddate.com/calendar/gregorian-calendar.html">Gregorian Calendar</a>
%o A231236 (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;break));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)&&((5*(y\4)+(y%4)-(y\100)+(y\400)-!(y%4)+!(y%100)-!(y%400))%7)==((5*((y+i)\4)+((y+i)%4)-((y+i)\100)+((y+i)\400)-!((y+i)%4)+!((y+i)%100)-!((y+i)%400))%7),j=2;break));if(j!=1,print1(i", ")))
%Y A231236 Cf. A230995-A231014, A231236-A231239.
%Y A231236 Cf. A231237 (Julian calendar).
%K A231236 nonn,easy
%O A231236 1,3
%A A231236 _Aswini Vaidyanathan_, Nov 06 2013