In number theory, Lucas's theorem expresses the remainder of division of the binomial coefficient by a prime number p in terms of the base p expansions of the integers m and n.
Lucas's theorem first appeared in 1878 in papers by Édouard Lucas. [1]
For non-negative integers m and n and a prime p, the following congruence relation holds:
where
and
are the base p expansions of m and n respectively. This uses the convention that if m < n.
There are several ways to prove Lucas's theorem.
Let M be a set with m elements, and divide it into mi cycles of length pi for the various values of i. Then each of these cycles can be rotated separately, so that a group G which is the Cartesian product of cyclic groups Cpi acts on M. It thus also acts on subsets N of size n. Since the number of elements in G is a power of p, the same is true of any of its orbits. Hence, modulo p equals the number of sets N whose orbit is of size 1, i.e., the number of fixed points of this group action. The fixed points are those subsets N that are a union of some of the cycles. This means that N must have exactly ni cycles of size pi for each i, for the same reason that the integer n has a unique representation in base p. Thus the number of choices for N is exactly .
This proof is due to Nathan Fine. [2]
If p is a prime and n is an integer with 1 ≤ n ≤ p − 1, then the numerator of the binomial coefficient
is divisible by p but the denominator is not. Hence p divides . In terms of ordinary generating functions, this means that
Continuing by induction, we have for every nonnegative integer i that
Now let m be a nonnegative integer, and let p be a prime. Write m in base p, so that for some nonnegative integer k and integers mi with 0 ≤ mi ≤ p − 1. Then
as the representation of n in base p is unique and in the final product, ni is the ith digit in the base p representation of n. This proves Lucas's theorem.
One consequence of Lucas's theorem is that the binomial coefficient is divisible by the prime p if and only if at least one of the digits of the base-p representation of n is greater than the corresponding digit of m. In particular, is odd if and only if the binary digits (bits) in the binary expansion of n are a subset of the bits of m.
Lucas's theorem can be generalized to give an expression for the remainder when is divided by a prime power pk. However, the formulas become more complicated.
If the modulo is the square of a prime p, the following congruence relation holds for all 0 ≤ s ≤ r ≤ p − 1, a ≥ 0, and b ≥ 0:
where is the nth harmonic number. [3] Generalizations of Lucas's theorem for higher prime powers pk are also given by Davis and Webb (1990) [4] and Granville (1997). [5]
Kummer's theorem asserts that the largest integer k such that pk divides the binomial coefficient (or in other words, the valuation of the binomial coefficient with respect to the prime p) is equal to the number of carries that occur when n and m − n are added in the base p.
There is a generalization of Lucas's theorem for the q-binomial coefficients. It asserts that if a, b, r, s, k are integers, where 0 ≤ b, s < k, then where and are q-binomial coefficients, is a usual binomial coefficient, and is the kth cyclotomic polynomial (in the variable q). [6]