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Is there a way to find the first digits of a number?

For example, the largest known prime is $2^{43,112,609}-1$, and I did sometime before a induction to find the first digit of a prime like that. But, is there a way to find the first digits of a number?

To find the last x digits is easy, just calculate it mod $10^x$, but we can do something about the first ones?

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Out of curiosity, why would you want to know the first digit? – JavaMan Dec 7 '11 at 22:45
It's the middle ones that are difficult. – Ross Millikan Dec 7 '11 at 23:02
@JavaMan, well, is just curiosity too, but I think it can be applied to study some numbers ^^. – GarouDan Dec 7 '11 at 23:05
@RossMillikan, I think you're right. – GarouDan Dec 7 '11 at 23:05
up vote 9 down vote accepted

What you want is $10$ to the power the fractional part of $43,112,609 \log_{10}2\approx 0.50033$, then $10^.50033\approx 3.1646$ so the leading digits are $316.$ Wolfram Alphaconfirms $31647$

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Thx Ross, this works. Unfortunally André had deleted his answer. Thx André too. – GarouDan Dec 7 '11 at 23:22
You just need to be careful and make sure that the rounding errors involved are not too much. Getting the first digit of pi raised to the 10^18th power would be hard. – gnasher729 Sep 22 '14 at 12:03

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