>>13046416You can see it is divisible by 3 since the sum of digits is 21.
However, this same sum also shows that our number is not divisible by 9, when it ought to be.
To make square tests faster, you divide by some primes to determine if
> The remainder is in-fact a quadratic residue> The greatest power of the prime which divides the number is evenAll exact square tests reduce down to trial and error at some point.
Inexact square tests are fine if you have a sufficiently accurate guess with something like Newton's method and just take a sqrt like
>>13046429 anon did.
