Friday, July 22, 2016

elementary number theory - Compute largest integer power of $6$ that divides $73!$



I am looking to compute the largest integer power of $6$ that divides $73!$



If it was something smaller, like $6!$ or even $7!$, I could just use trial division on powers of $6$. However, $73!$ has $106$ decimal digits, and thus trial division isn't optimal.



Is there a smarter way to approach this problem?


Answer




HINT: There are $\lfloor73/3\rfloor=24$ numbers divisible by 3, $\lfloor73/9\rfloor=8$, numbers divisible by 9, $\lfloor73/27\rfloor=2$ numbers divisible by 27 in the set $[1,73]\cap\mathbb{N}$. It should be easy now to obtain that the answer is 34 (with the value $6^{34}$).


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