cp's OEIS Frontend

A230995 Number of years after which a date can fall on the same day of the week, in the Gregorian calendar.

%I A230995 #5 Nov 02 2013 23:37:39
%S A230995 0,5,6,7,11,12,17,18,22,23,28,29,33,34,35,39,40,45,46,50,51,56,57,61,
%T A230995 62,63,67,68,73,74,78,79,84,85,89,90,91,95,96,101,102,106,107,108,112,
%U A230995 113,114,117,118,119,123,124,125,129,130,131,134,135,136,140,141,142
%N A230995 Number of years after which a date can fall on the same day of the week, in the Gregorian calendar.
%C A230995 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 A230995 Assuming this fact, this sequence is periodic with a period of 400.
%H A230995 Time And Date, <a href="http://www.timeanddate.com/calendar/repeating.html">Repeating Calendar</a>
%H A230995 Time And Date, <a href="http://www.timeanddate.com/calendar/gregorian-calendar.html">Gregorian Calendar</a>
%e A230995 5 belongs to this sequence because January 1, 2008 falls on same day as January 1, 2013.
%e A230995 6 belongs to this sequence because January 1, 2009 falls on same day as January 1, 2015.
%e A230995 7 belongs to this sequence because January 1, 2097 falls on same day as January 1, 2104.
%o A230995 (PARI) for(i=0,400,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),print1(i", ");break)))
%Y A230995 Cf. A230995-A231014.
%Y A230995 Cf. A231000 (Julian calendar).
%K A230995 nonn,easy
%O A230995 0,2
%A A230995 _Aswini Vaidyanathan_, Nov 02 2013