Posted by: Gary Ernest Davis on: May 6, 2011

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Benjamin Vitale (@BenVitale)

Alex Bogomolny (@CutTheKnotMath) observed that if (digits base 10) then has a factor of unless one of .

A quick computational search (using Mathematicaâ„¢) yields the following numbers (base 10 digits) for which is prime:

10, 20, 203, 230, 308, 309, 330, 350, 503, 603, 650, 960, 1068, 1110, 1206, 1350, 1404, 1480, 1730, 1802, 1860, 1910, 2032, 2038, 2044, 2054, 2250, 2320, 2502, 3044, 3082, 3402, 3970, 4032, 4046, 4072, 4120, 4340, 4450, 4540, 4650, 4908, 5204, 5310, 5402, 5530, 5770, 5820, 6038, 6076, 6120, 6206, 6490, 6520, 6830, 7110, 8074, 8106, 8120, 8380, 8450, 8610, 8690, 8902, 8960, 9064, 9330, 9402, 9408, 9950

How do these numbers increase?

Are they becoming relatively rarer?

Let’s denote by the number of such that Â is prime.

How does the fraction vary with increasing ?

Here is a plot of that fraction up to :

It appears that as increases, although the decrease is highly exaggerated by the foreshortened plot.

Does as increases?

Matt Henderson (@mattthen2) made the following useful observation:

Let denote the number of whose digits contain a .

Then so for all .

Because for all we see that:

for all .

Tags: prime numbers

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