A 10-digit number satisfying the following property. Number the digits 0 to 9, and let digit 
be the number of 
s in the number. There is exactly
one such number: 6210001000.
Self-Descriptive Number
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References
Pickover, C. A. "Chaos in Ontario." Ch. 28 in Keys to Infinity. New York: Wiley, pp. 217-219, 1995.Referenced on Wolfram|Alpha
Self-Descriptive NumberCite this as:
Weisstein, Eric W. "Self-Descriptive Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Self-DescriptiveNumber.html