29999999999

This number is a composite.

                                                                2 9999999999

The largest “d-sphenic number” obtained by inserting d d's, (where d is a digit), between the digits of a 2-digit prime, (nine 9's between the digits of 29). A “d-sphenic number” is a sphenic number whose prime factors have the same length d, i.e., 29999999999 = 2113 × 3767 × 3769. Note that concatenating in some order its prime factors we create an emirp, i.e., 376737692113. [Loungrides]