tribonacci v1.0.2

Tribonacci function

Tribonacci function as defined in the OEIS A000073 article.

This is a recursive function that works the following way:

tribonacci(0n) // => 0n
tribonacci(1n) // => 0n
tribonacci(2n) // => 1n
tribonacci(3n) // => 1n, as a result of tribonacci(2n) + tribonacci(1n) + tribonacci(0n)
// ...
tribonacci(9n) // => 44n, as a result of tribonacci(8n) + tribonacci(7n) + tribonacci(6n)