# Number of Digits in Power of 2/Examples/Mersenne Number M127

## Example of Number of Digits in Power of 2

When expressed in conventional decimal notation, the number of digits in the Mersenne number $M_{127}$ is $39$.

## Proof

Let $m$ be the number of digits in the Mersenne number $M_{127}$.

Recall the definition Mersenne number $M_{127}$:

$M_{127} = 2^{127} - 1$

We have that $2^{127}$ is not a power of $10$.

Neither can $2^{127} - 1$ be a power of $10$.

So $M_{127}$ and $2^{127}$ have the same number of digits.

$m = \ceiling {127 \log_{10} 2}$
$\log_{10} 2 \approx 0 \cdotp 30102 \, 99956 \, 63981 \, 19521 \, 37389 \ldots$

and so:

$m = \ceiling {38 \cdotp 23}$

Hence the result.

$\blacksquare$