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Years with exactly one "Friday the 13th", starting from 1901.
4

%I #16 Feb 16 2025 08:33:14

%S 1902,1904,1909,1910,1913,1915,1916,1919,1921,1924,1926,1927,1930,

%T 1932,1937,1938,1941,1943,1944,1947,1949,1952,1954,1955,1958,1960,

%U 1965,1966,1969,1971,1972,1975,1977,1980,1982,1983,1986,1988,1993,1994,1997,1999,2000

%N Years with exactly one "Friday the 13th", starting from 1901.

%H Reinhard Zumkeller, <a href="/A190651/b190651.txt">Table of n, a(n) for n = 1..1000</a>

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Triskaidekaphobia.html">Triskaidekaphobia</a>

%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Triskaidekaphobia">Triskaidekaphobia</a>

%H <a href="/index/Ca#calendar">Index entries for sequences related to calendars</a>

%F A101312(a(n)) = 1, 1 <= A101312(n) <= 3.

%e 1902 is a term, since only Jun 13 1902 fell on a Friday.

%t Select[Range[1901,2001],Count[Table[DayName@{#,m,13},{m,12}],Friday]==1&] (* _Giorgos Kalogeropoulos_, Sep 12 2021 *)

%o (Haskell)

%o a190651 n = a190651_list !! (n-1)

%o a190651_list = filter ((== 1) . a101312) [1901..]

%o (Python)

%o from datetime import date

%o def ok(n): return sum(date.isoweekday(date(n, m, 13)) == 5 for m in range(1, 13)) == 1

%o print(list(filter(ok, range(1901, 2001)))) # _Michael S. Branicky_, Sep 12 2021

%Y Cf. A101312, A190652, A190653.

%K nonn,changed

%O 1,1

%A _Reinhard Zumkeller_, May 16 2011