# Large Number

• July 10th 2009, 06:30 PM
AlephZero
Large Number
What is the last (rightmost) digit of

$23^{23^{23^{23}}}$?
• July 10th 2009, 07:53 PM
TwistedOne151
The ones digits of powers of 23 cycle as do the ones digits of powers of 3, and as 3x3=9, 3x9=27, 3x7=21, 3x1=3, we see that the ones digits cycle through the values 3, 9, 7, 1. Thus, to find the ones digit of $23^{23^{23^{23}}}$, we need only find $23^{23^{23}}$ mod 4. Now, 23 mod 4 = 3, and so we see that
$23^2$ mod 4 = 1, and thus $23^n$ mod 4 is 3 when n is odd and 1 when n is even. And we see that since 23 is odd, $23^{23}$ is odd, and so $23^{23^{23}}$ mod 4 = 3, and thus the ones digit of $23^{23^{23^{23}}}$ is the same as that of $3^3$, which is 7.

--Kevin C.
• July 10th 2009, 08:02 PM
AlephZero
Lovely.