# Quick Divisibility Test for Primes up to 67

While there’s known divisibility tests for certain primes (see my notes), the following test very quickly determines if a number is divisible by any prime up to 67 by hand! The original method is due to John Conway, and I learned it from Arthur Benjamin.

Here’s a sample problem along with explanation:

63779 is the product of three primes, each less than $71$. Determine their sum.

We set up the equation $63779=pqr$, where $p, q,$ and $r$ are primes.

We begin by finding the factor of 2000 closest to 63, namely 64,000. We can therefore rewrite this as

$221=2000\cdot 32-63,779.$

We can quickly determine that since $2, 5\mid 2000$ and $2, 5\nmid 221$, $2, 5\nmid 63,779$. We now add $32$, the quotient in the divisibility above to both sides of the equation to produce:

$253=2001\cdot 32-63,779.$

Notice the factor of $32$ and the left hand side have both increased, while our original number, 63779 has remained the same. Now, as any good contest math student should have the years close to the current year memorized, we know $2001=3\cdot 23\cdot 29.$ Therefore $3\mid 63779\iff 3\mid 253$, however, $3\nmid 253$. Similarly $23\mid 63779\iff 23\mid 253$. This time, $253=23\cdot 11$, therefore $23\mid 63779$. We’ve found one prime divisor: $p=23$.

We could now divide $63779$ by $23$, however, this gives the equation $2773=qr$, and we don’t have any better ideas for how to factorize $2773$. Therefore, instead, we’ll use the prime factorizations of some other numbers close to $2000$:

$1998=2\cdot 3^3\cdot 37, 2002=2\cdot 7\cdot 11\cdot 13, 2006=2\cdot 17\cdot 59, 2009=41\cdot 49$

and $2010=2\cdot 3\cdot 5\cdot 67,2013=3\cdot 11\cdot 61, 2015=5\cdot 13\cdot 31, 2021=43\cdot 47.$

For instance, to determine if our number is divisible by $37$, we would take the original equation and subtract $2\cdot 32$ to produce $157=1998\cdot 32-63,779.$ Then, we know $37\mid 63779\iff 37\mid 157$, however, clearly $37\nmid 157$.

Similarly, to test if our number is divisible by $7, 11,$ or $13$, we simply add $2\cdot 32$ to our original equation to produce $285=2002\cdot 32-63779$. Therefore, again $11\mid 285\iff 11\mid 63779$, however, clearly $11\nmid 285$, hence the same is true for $63779$.

Next, to determine if the original number is divisible by $17$ or $59$, we add $6\cdot 32$ to our original equation (or more simply, $4\cdot 32$ to the equation in the last paragraph) to get

$413=2006\cdot 32-63779.$

Since $2006=2\cdot 17\cdot 59$, we see $17\mid 413\iff 17\mid 63779$, however, clearly $17\nmid 413$. Similarly, $59\mid 413\iff 59\mid 63779$. Now since $420=60\cdot 7$, we see $413=59\cdot 7$, hence $59\mid 413$, and $59\mid 63779$. We’ve therefore found a second prime factor: $q=59$.

At this point that we have two prime factors, we could continue on with our test, but we have a pretty good idea of what the third factor is by division: $63779=23\cdot 59\cdot r$. One way to quickly compute this in our heads would be to notice

$59\cdot 23=(60-1)(20+3)=1200+180-20-3=1357.$

Since we know $r$ is an integer, we attempt to bound it. We know $r=\frac{63779}{1357}$. We notice $40\cdot 1357=52480$, while $50\cdot 1357=67850$, therefore, $40.  We then subtract $40$ from both sides of our equation:

$r-40=\frac{63779-52480}{1357}.$

While we could compute the numerator mentally, since we know $1\le r-40\le 9$, we know it must be a single numerical digit! We therefore just consider the unit digit of the numerator which is $9$. Since we’re dividing by a number which ends in $7$, we know $r-40$ must be $7$, since $7\cdot 7\equiv 49\equiv 9\pmod{10}$. Therefore $r=47$.

Hence our desired sum is $p+q+r=23+59+47=\boxed{129}$.

Alternatively, we could verify $47\mid 63779$ using the prime factorization of next year, 2021. I’ll leave this as an exercise for anyone interested.

While this method isn’t super practical, I think it’s interesting that you can essentially compute prime factors up to 67 with a quick trick in your head. The most difficult part is memorizing the prime factorizations of the numbers close to 2000, but hopefully this is familiar for any seasoned contest mathematician (who might be interested in using this).

References: Arthur Benjamin, Factoring Numbers with Conway’s 150 Method
The College Mathematics Journal, Vol. 49, No. 2, pp. 122-125, March 2018.