13

This number is a prime.

There are thirteen prime outputs among the first one hundred terms in the Tribonacci sequence TRI (n) which is recursively defined as follows: TRI (1) = TRI (2) = TRI (3) = 1 and TRI (n) = TRI (n-3) + TRI (n-2) + TRI (n-1) for n>=4. The thirteen prime outputs 3, 5, 17, 31, 193, 653, 1201, 46499, 532159, 3311233, 128199521, 20512526282340991, and 6036724301884645488192191 corresponding to the fourth, fifth, seventh, eighth, eleventh, thirteenth, fourteenth, twentieth, twenty-fourth, twenty-seventh, thirty-third, sixty-fourth, and ninety-sixth terms respectively. [Schiffman]