Tribonacci Day - July 13

You may have heard of the Fibonacci numbers, which starts with 0 and 1, and where every following number is the sum of the previous 2 numbers. Thus Fibonacci numbers are 0, 1, 0+1 = 1, 1+1=2, 1+2=3, 2+3=5, 3+5=8, ...

Mathematicians quickly noticed that they could define a variety of similar sets of numbers by changing the rules slightly. Perhaps the next easiest to deal with are the Tribonacci numbers (named by merging the tri- suffix with much of the original Fibonacci name). The Tribonacci numbers start with the "seed numbers" of 0, 1, and 1, and each of the following numbers are the sum of the previous 3 numbers. Thus we get numbers of 0, 0, 1, 0+0+1 = 1, 0+1+1=2, 1+1+2=4, 1+2+4=7, ...

While the Fibonacci numbers are known to show up in nature, the Tribonacci numbers are mostly of interest to mathematicians alone. (The ratio between adjacent Tribonacci numbers - the Tribonacci constant - is used in calculations involving the snub cube.) We celebrate Tribonacci numbers on 7/13 because those are the largest sequential Tribonacci numbers that represent a (Gregorian calendar) date.