Tribonacci Word

Tribonacci word. is a specific sequence of digits. The Tribonacci word is formed by repeated concatenation in the same way that the Fibonacci word is formed by repeated addition. But unlike the fibonacci word, Tribonacci word is formed by repeated addition of last three terms and it has its first three terms different from each other.
 

In Tribonacci word,
 S(0) = 1, 
 S(1) = 12, 
 S(2) = 1213,
 S(3) = 1213121 
 ..... 
where S(n) = S(n-1) + S(n-2) + S(n-3) and + 
represents the concatenation of 
strings.

Tribonacci Word : https://www.geeksforgeeks.org/tribonacci-word/