24444...99991 (29-digits)

This number is a prime.

                                             244446666 6688888888 9999999991

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The only prime formed by inserting in ascending order, between the digits of a double-digit composite number n, (n=21), the concatenation of the composite digits d, d times each of them, i.e., four 4’s, six 6’s, eight 8’s and nine 9’s. [Loungrides]