The black hole number 4 has been previously discussed here.  Below is a look at the black hole number 123 (click the image for a larger view):

*Another interesting note: In Italian, the number 3 (tre) is a black hole number.

Recently, Cliff Pickover tweeted about the number 4 being the only number in English have the same number of letters as its value.  I tweeted a response that in Danish this is true for the numbers 2, 3, and 4 (to, tre, fire). These numbers are often referred to as honest numbers, which are related to black hole numbers. Also, there are honest expressions such as this one:

This honest expression was created by Michael Burke.

References:

Ecker, M. Number play, calculators, and card tricks: Mathemagical black holes. Retrieved from http://g4gardner.pbworks.com/f/mm-ecker.pdf

Gardner, M. (1976). The incredible dr. matrix. New York: Charles Scribner’s Sons.

O’Shea, O. & Dudley, U. (2007). The magic numbers of the professor. Washington, DC: The Mathematical Association of America, Inc.