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 A231007 #4 Nov 02 2013 23:40:08 %S A231007 1,11,19,27,28,44,45,53,61,70,71,73,74,83,91,99,100,116,125,131,133, %T A231007 143,145,146,160,171,177,185,193,202,203,205,206,215,217,223,231,232, %U A231007 248,249,257,263,265,274,275,277,278,287,295,303,309,320,334,335,337,347,349,355,364 %N A231007 Number of months after which a date can fall on the same day of the week, but it is not possible that the two months have the same number of days, in the Gregorian calendar. %C A231007 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 A231007 Assuming this fact, this sequence is periodic with a period of 4800. %C A231007 These are the terms of A231005 not in A231006. %H A231007 Time And Date, <a href="http://www.timeanddate.com/calendar/repeating-month.html">Repeating Months</a> %H A231007 Time And Date, <a href="http://www.timeanddate.com/calendar/gregorian-calendar.html">Gregorian Calendar</a> %e A231007 1 belongs to this sequence because February 1, 2013 falls on the same day as March 1, 2013, but both February and March do not have the same number of days. In fact, a difference of 1 month can never produce the same calendar for the entire month, with the same number of days. %e A231007 11 belongs to this sequence because December 1, 2011 falls on the same day as November 1, 2012 but both December and November do not have the same number of days. In fact, a difference of 11 months can never produce the same calendar for the entire month, with the same number of days. %o A231007 (PARI) m=[0,3,3,6,1,4,6,2,5,0,3,5];n=[31,28,31,30,31,30,31,31,30,31,30,31];y=vector(4800,i,(m[((i-1)%12)+1]+((5*((i-1)\48)+(((i-1)\12)%4)-((i-1)\1200)+((i-1)\4800)-!((i-1)%48)+!((i-1)%1200)-!((i-1)%4800)-!((i-2)%48)+!((i-2)%1200)-!((i-2)%4800))))%7);x=vector(4800,i,n[((i-1)%12)+1]+!((i-2)%48)-!((i-2)%1200)+!((i-2)%4800));for(p=0,4800,j=0;for(q=0,4800,if(y[(q%4800)+1]==y[((q+p)%4800)+1],j=1;break));for(q=0,4800,if(y[(q%4800)+1]==y[((q+p)%4800)+1]&&x[(q%4800)+1]==x[((q+p)%4800)+1],j=2;break));if(j==1,print1(p", "))) %Y A231007 Cf. A230995-A231014. %Y A231007 Cf. A231012 (Julian calendar). %K A231007 nonn,easy %O A231007 1,2 %A A231007 _Aswini Vaidyanathan_, Nov 02 2013