Divisibility of 9 trick: why does it work?
3128
Why $9$ & $11$ are special in divisibility tests using decimal digit sums? (casting out nines & elevens)
146947
A multiplication algorithm found in a book by Paul Erdős: how does it work?
124136
Why does the division algorithm work for converting between number bases?
126587
Divisiblity of binomial coefficients
36182
Why doesn't the last digit method work for divisibility by 4?
73050
Explanation of the binomial theorem and the associated Big O notation
50662
Why does this pattern occur: $123456789 \times 8 + 9 = 987654321$
129693
An interesting problem with "decomposing" natural numbers.
110444
Finding out the number of divisors
139380
Fibonacci sequence divisible by 7?
5183
Divisibility of determinant.
140087
The number of divisors
91178
chain of divisibility relation
140364
Covering ten dots on a table with ten equal-sized coins: explanation of proof
78946
Why does casting out $9$ and $11$ work to compute remainders?
31708
Divisibility criteria for $7,11,13,17,19$
126124
Why is the sum of the digits in a multiple of 9 also a multiple of 9?