A008685 - OEIS

A008685

Lengths of months in the Gregorian calendar.

31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 28, 31

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

To make the definition unambiguous, consider these as starting from January 2001 (or, equivalently, AD 1 in the proleptic Gregorian calendar). [Charles R Greathouse IV, Sep 28 2011]

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Index entries for sequences related to calendars

Index entries for linear recurrences with constant coefficients, order 4800.

FORMULA

a(n) = a(n-4800). [Charles R Greathouse IV, Sep 28 2011]

MATHEMATICA

year1 = 2001; year2 = 2100;

PreviousDate[#, "Day"][[1, 3]]& /@ Flatten[Table[{#, m, 1}, {m, Append[ Range[2, 12], 1]}]& /@ Range[year1, year2], 1] (* Jean-François Alcover, Aug 01 2018 *)

PROG

(PARI) v=[]; for(n=1, 50, v=concat(v, [31, if(n%4||(n%100==0&&n%400), 28, 29), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31])); v

(Haskell)

a008685 n = a008685_list !! (n-1)

a008685_list = concatMap t [1..] where

t y = [31, 28 + leap, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

where leap = if mod y 4 == 0 &&

(mod y 100 > 0 || mod y 400 == 0) then 1 else 0

-- Reinhard Zumkeller, Jun 19 2015

(Python)

def a(n):

y, m = 1 + (n-1)//12, (n-1)%12