Freshman's Dream
Lesson · Advanced
Number Theory: Modular Arithmetic
Freshman's Dream
If p is prime, then
More generally
Repeated Freshman's Dream
For every positive integer k
By the Binomial Theorem
Since p is prime, p divides the numerator but does not divide k!(p-k)! Therefore
for every 1≤k≤p−1
Thus all the middle terms vanish modulo p, giving
It can also be proved rigorously by induction on n
It can also be proved rigorously by induction on k
For integers a
Therefore, Freshman's Dream also gives
More generally
Find the remainder when
is divided by 3
Therefore every intermediate coefficient is divisible by 3
Since 81 is strictly between 0 and 243
Simplify
a is an integer
Find