1111111111111111111 (another Prime Pages' Curiosity)
Curios: Curios Search:

Participate:
```                                                        111111111 1111111111 ```
```      1 1 1
1 1 1 1
1 1 1 1 1
1 1 1 1
1 1 1
```

A repunit hex-congruent prime in bases 11 and 12. [Dobb]

The largest prime factor of the alternate-digit, palindromic semiprime 1010101010101010101010101010101010101 is the repunit prime 1111111111111111111. Both numbers contain 19 1's. [Green]

The second repunit prime. [Gronos]

1111111111111111111^2 = 1111111111111111111 (mod 11111111111111111111111111111111111111). [Luhn]

The second (and largest known) repunit prime to be prime in both decimal and binary. [Green]

 Each of the 10 palindromes in the trapezoidal structure to the right contains 19 ones, and the repunit prime, R19, is the largest prime factor of each one. 1111111111111111111 101111111111111111101 10101111111111111110101 1010101111111111111010101 101010101111111111101010101 10101010101111111110101010101 101010101010111111101010101010 101010101010101111101010101010101 10101010101010101110101010101010101 1010101010101010101010101010101010101
[Green]

The seventh Mersenne prime (219-1) written in binary.

The smallest prime Kaprekar number. E.g., 1111111111111111111^2 = 1234567901234567900987654320987654321 and 123456790123456790 + 0987654320987654321 = 1111111111111111111. [Rivera]

(There are 6 curios for this number that have not yet been approved by an editor.)