A190652 Years with exactly two "Friday the 13ths", starting from 1901.

1901, 1905, 1906, 1907, 1908, 1911, 1912, 1917, 1918, 1920, 1922, 1923, 1929, 1933, 1934, 1935, 1936, 1939, 1940, 1945, 1946, 1948, 1950, 1951, 1957, 1961, 1962, 1963, 1964, 1967, 1968, 1973, 1974, 1976, 1978, 1979, 1985, 1989, 1990, 1991, 1992, 1995, 1996

Offset

1,1

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..1000

Eric Weisstein's World of Mathematics, Triskaidekaphobia

Wikipedia, Triskaidekaphobia

Index entries for sequences related to calendars

Formula

A101312(a(n)) = 2, 1 <= A101312(n) <= 3.

Example

2004 is a term, since only Feb 13 2004 and Aug 13 2004 fell on a Friday.

Mathematica

Select[Range[1901, 2020], Count[Table[{#, m, 13}, {m, 12}], _?(DayName[#] == Friday&)] == 2&] (* Harvey P. Dale, Oct 02 2018 *)

Prog

(Haskell)
a190652 n = a190652_list !! (n-1)
a190652_list = filter ((== 2) . a101312) [1901..]
(Python)
from datetime import date
def ok(n): return sum(date.isoweekday(date(n, m, 13)) == 5 for m in range(1, 13)) == 2
print(list(filter(ok, range(1901, 2000)))) # Michael S. Branicky, Sep 12 2021

Crossrefs

Cf. A101312, A190651, A190653.

Sequence in context: A204479 A251346 A078862 * A177137 A273913 A190651

Adjacent sequences: A190649 A190650 A190651 * A190653 A190654 A190655

Keyword

nonn

Author

Reinhard Zumkeller, May 16 2011

Status

approved