This number is a prime.

                                                        112233445 5667788991

+ Place 1 after the concatenation of the 9 digits duplicated and obtain a 19-digit prime, whose sum of digits is the 91 appearing the end of the prime. [Silva]

+ (1122334455667788991, 1122334455667788997) is a sexy prime pair formed by doubling each counting digit in order and attaching at the end respectively the digits 1 and 7. [Loungrides]

(There is one curio for this number that has not yet been approved by an editor.)

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