The tribonacci numbers are a generalization of the Fibonacci numbers defined by 
, 
, 
, and the recurrence
equation
 |
(1)
|
for 
(e.g., Develin 2000). They represent the 
case of the Fibonacci
n-step numbers.
The first few terms using the above indexing convention for 
, 1, 2, ... are 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, ...
(OEIS A000073; which however adopts the alternate
indexing convention 
and 
).
The first few prime tribonacci numbers are 2, 7, 13, 149, 19341322569415713958901, ... (OEIS A092836), which have indices 3, 5,
6, 10, 86, 97, 214, 801, 4201, 18698, 96878, ... (OEIS A092835),
and no others with 
(E. W. Weisstein, Mar. 21, 2009).
Using Brown's criterion, it can be shown that the tribonacci numbers are complete; that is, every positive number can be written as the sum of distinct tribonacci numbers. Moreover, every positive number has a unique Zeckendorf-like expansion as the sum of distinct tribonacci numbers and that sum does not contain three consecutive tribonacci numbers. The Zeckendorf-like expansion can be computed using a greedy algorithm.
An exact expression for the 
th tribonacci number can be given explicitly by
 |  |  |
(2)
|
 |  |  |
(3)
|
where 
are the three roots of the polynomial
 |
(4)
|
This can be written in slightly more concise form as
 |
(5)
|
where 
is the 
th
root of the polynomial
 |
(6)
|
and 
and 
are in the ordering of the Wolfram Language's
Root object.
The tribonacci numbers can also be computed using the generating function
 |
(7)
|
Another explicit formula for 
is also given by
![[3({1/3(19+3sqrt(33))^(1/3)+1/3(19-3sqrt(33))^(1/3)+1/3}^n(586+102sqrt(33))^(1/3))/((586+102sqrt(33))^(2/3)+4-2(586+102sqrt(33))^(1/3))],](/images/equations/TribonacciNumber/NumberedEquation6.svg) |
(8)
|
where ![[x]](/images/equations/TribonacciNumber/Inline23.svg)
denotes the nearest integer function (Plouffe).
The first part of the numerator is related to the real root of 
, but determination of the denominator
requires an application of the LLL algorithm.
The ratio of adjacent terms tends to the positive real root 
, namely 1.83929... (OEIS A058265),
sometimes known as the tribonacci constant.
By considering the series 
(mod 
), one can prove that any integer 
is a factor of 
for some 
(Brenner 1954). The smallest values of 
for which 
is a factor for 
, 2, ... are given by 1, 3, 7, 4, 14, 7, 5, 7, 9, 19, 8,
7, 6, ... (OEIS A112305).
The tribonacci constant is extremely prominent in the properties of the snub cube, its dual the pentagonal icositetrahedron, and the snub cube-pentagonal icositetrahedron compound. It can even be used to express the hard hexagon entropy constant.
With different initial values, the tribonacci sequence starts as 
, 
, 
, 
, 
, 
, 
, 
, ..., which gives the following sequences as special
cases.
 |  |  | OEIS | sequence |
| 0 | 0 | 1 | A000073 | 0, 1, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, ... |
| 1 | 1 | 1 | A000213 | 1, 1, 1, 3, 5, 9, 17, 31, 57, 105, 193, 355, ... |
| 0 | 1 | 0 | A001590 | 0, 1, 0, 1, 2, 3, 6, 11, 20, 37, 68, 125, 230, ... |
| 3 | 1 | 3 | A001644 | 3, 1, 3, 7, 11, 21, 39, 71, 131, 241, 443, 815, ... |
 | 2 | 2 | A100683 |  ,
2, 2, 3, 7, 12, 22, 41, 75, 138, 254, 467, ... |
