# Definition:Tribonacci Constant

## Definition

The Tribonacci constant $\eta$ is the one real root of the cubic:

$x^3 - x^2 - x - 1 = 0$

Its decimal expansion starts:

$\eta = 1 \cdotp 83928 \, 67552 \, 1416 \ldots$

## Also known as

Some sources leave its first letter in lowercase: tribonacci constant.

## Linguistic Note

The word Tribonacci, in the context of Tribonacci constant and Tribonacci sequence, is a portmanteau word formed from tri, from the Greek word for three, and the name of the mathematician Fibonacci.

Hence it is pronounced trib-bo-nat-chi, or trib-bo-nar-chi, according to taste.

The word arises as a direct analogy with the Fibonacci numbers.