*a mathematical riddle from my inbox*

Earlier this year I received a cheerful, pleasant riddle from a cheerful, pleasant fellow named Simon.

Today, as my mother (born in 1947) turns 74, I realized that I (born 1983) will turn 38 this year. Investigating it further, it turns out that this year everybody whose sum of digits (last two digits) of year of birth is 11 gets this result. Examples:

BornAge in 20211929 92

1938 83

1947 74

1956 65

1965 56

1974 47

1983 38

1992 29How is that? I fail to find the mathematic principle that leads to this result. Would you care to help?

First: Happy belated birthday to Simon’s mother!

Second: What a nifty observation. Exactly the kind of head-scratcher I love to bring into the classroom.

Admittedly, facts about digits rarely strike at deep mathematics. After all, the patterns vanish when translated out of base ten. But who cares? They’re a fun playground for conjectures, and good practice at teasing out “funny coincidence” from “necessary consequence.”

So let me pose my own question, a riff on Simon’s: **How special is this year for digit-swapping birthdays?**

Spoilers follow!

Everybody will have a digit-swapping birthday. Let’s say the last two digits of your birthyear are **ab** (as in **19ab** or **20ab** or, if you are the oldest person on earth, **18ab**). In that case, your digit-swapping birthday occurs when you turn **ba** years old.

In what year does this happen? Well, it works like this:

- Begin with the century of your birth (1900 or 2000).
- Add
**a**decades and**b**years (to get to your birth year). - Add
**b**decades and**a**years (to get to your digit-swapping year).

Thus, if we define **n** as **a+b**, your digit-swapping year is **[century] + n decades + n years**.

If **n < 10**, then things are pretty simple. Your digit-swapping birthday happens in the same century as your birth, in a year of the form **19nn** or **20nn**.

N | Digit Swapping Year | Birth Years |

1 | 1911 | 1901, 1910 |

2 | 1922 | 1902, 1911, 1920 |

3 | 1933 | 1903, 1912, 1921, 1930 |

4 | 1944 | 1904, 1913, 1922, 1931, 1940 |

5 | 1955 | 1905, 1914, 1923, 1932, 1941, 1950 |

6 | 1966 | 1906, 1915, 1924, 1933, 1942, 1951, 1960 |

7 | 1977 | 1907, 1916, 1925, 1934, 1943, 1952, 1961, 1970 |

8 | 1988 | 1908, 1917, 1926, 1935, 1944, 1953, 1962, 1971, 1980 |

9 | 1999 | 1909, 1918, 1927, 1936, 1945, 1954, 1963, 1972, 1981, 1990 |

But what **n > 9**? Then your digit-swapping birthday will land in the century after your birth, and will occur in a year of the form **20(n – 9)(n -10)**.

N | Digit Swapping Year | Birth Years |

10 | 2010 | 1919, 1928, 1937, 1946, 1955, 1964, 1973, 1982, 1991 |

11 | 2021 | 1929, 1938, 1947, 1956, 1965, 1974, 1983, 1992 |

12 | 2032 | 1939, 1948, 1957, 1966, 1975, 1984, 1993 |

13 | 2043 | 1949, 1958, 1967, 1976, 1985, 1994 |

14 | 2054 | 1959, 1968, 1977, 1986, 1995 |

15 | 2065 | 1969, 1978, 1987, 1996 |

16 | 2076 | 1979, 1988, 1997 |

17 | 2087 | 1989, 1998 |

18 | 2098 | 1999 |

In each century, there are 100 birth cohorts. Their digit-swapping birthdays come in clusters: 9 pairs of years, scattered across the century, in the form **20c(c-1) **and **20cc**, for c = 1 to 9. In each pair of years, exactly 11 cohorts celebrate digit-swapping birthdays.

First Year | Cohorts | Second Year | Cohorts |

2010 | 9 (turning 91, 82, 73, 64, 55, 46, 37, 28, 19) | 2011 | 2 (turning 1, 10) |

2021 | 8 (turning 92, 93, 74, 65, 56, 47, 38, 29) | 2022 | 3 (turning 2, 11, 20) |

2032 | 7 (turning 93, 84, 75, 66, 57, 48, 39) | 2033 | 4 (turning 3, 12, 21, 30) |

2043 | 6 (turning 94, 85, 76, 67, 58, 49) | 2044 | 5 (turning 4, 13, 22, 31, 40) |

2054 | 5 (turning 95, 86, 77, 68, 59) | 2055 | 6 (turning 5, 14, 23, 32, 41, 50) |

2065 | 4 (turning 96, 87, 78, 69) | 2066 | 7 (turning 6, 15, 24, 33, 42, 51, 60) |

2076 | 3 (turning 97, 88, 79) | 2077 | 8 (turning 7, 16, 25, 34, 43, 52, 61, 70) |

2087 | 2 (turning 98, 89) | 2088 | 9 (turning 8, 17, 26, 35, 44, 53, 62, 71, 80) |

2098 | 1 (turning 99) | 2099 | 10 (turning 9, 18, 27, 36, 45, 54, 63, 72, 81, 90) |

Hey, that’s only 99 cohorts. What about the final cohort?

Well, those born in 2000 don’t really get a digit-swapping birthday. Or perhaps they get two: one right away, in 2000, and another in 2100, if they can live to a century.

Finally, an answer to the riddle!

2021 has eight digit-swapping cohorts. That ties it with 2088 for third-most, trailing only the nine-cohort years of 2010 and 2099. That puts it at the 96th percentile of digit-swapping specialness. **In short: this year is more special than 24 out of 25 years**.

Pretty special!

My mother (born in 47, turned 74 this year) and I (born in 74, turning 47 this year) have simultaneous digit swapping birthdays this year which we think is pretty darn geeky cool!