Olympiad Maths Prep

Track / Stage 7 / 17 of 300 #1417 of 2000

Problem 1417

National olympiad second round; IMO P1/P4
Number theory Difficulty 7.0 Find the answer

Let r2,r3,,r1000r_2, r_3,\ldots, r_{1000} denote the remainders when a positive odd integer is divided by 2,3,,10002,3,\ldots,1000, respectively. It is known that the remainders are pairwise distinct and one of them is 00. Find all values of kk for which it is possible that rk=0r_k = 0.

Official solution

1. Let r2,r3,,r1000 r_2, r_3, \ldots, r_{1000} denote the remainders when a positive odd integer N N is divided by 2,3,,1000 2, 3, \ldots, 1000 , respectively. It is given that these remainders are pairwise distinct and one of them is 0 0 .

2. Assume NS N \in \mathbf{S} , where S \mathbf{S} is the set of integers satisfying the given conditions. Let p p be the unique integer for which rp=0 r_p = 0 .

3. If p p is composite, there exists a prime q<p q < p which divides p p , implying rq=0 r_q = 0 . This is a contradiction since the remainders are pairwise distinct. Therefore, p p must be a prime number.

4. Since N N is odd, r2=1 r_2 = 1 . This implies p2 p \neq 2 .

5. The fact that 0rkk1 0 \leq r_k \leq k-1 implies r3=2 r_3 = 2 , r4=3 r_4 = 3 , r5=4 r_5 = 4 , and so on, up to rp1=p2 r_{p-1} = p-2 . This gives us the sequence:
rk=k1for2kp1. r_k = k-1 \quad \text{for} \quad 2 \leq k \leq p-1.

6. Assume p<500 p < 500 . Then 2p<1000 2p < 1000 . Since rp=0 r_p = 0 , p p divides r2p r_{2p} , implying r2p=p r_{2p} = p because r2p<2p r_{2p} < 2p and r2prp=0 r_{2p} \neq r_p = 0 .

7. This combined with the sequence rk=k1 r_k = k-1 for 2kp1 2 \leq k \leq p-1 and the fact that rp+1{0,1,2,,p} r_{p+1} \in \{0, 1, 2, \ldots, p\} implies rp+1=p1 r_{p+1} = p-1 . However, rp+1 r_{p+1} must be odd since p p is an odd prime, leading to a contradiction. Therefore, p>500 p > 500 .

8. Next, assume 500<p<1000 500 < p < 1000 . For positive integers kp k \leq p , define the odd positive integer Nk=1000!pk1 N_k = \frac{1000!}{p}k - 1 .

9. Let m m be an integer such that 2m1000 2 \leq m \leq 1000 and mp m \neq p . Clearly, m m divides 1000!p \frac{1000!}{p} , hence:
Nk1m1(modm), N_k \equiv -1 \equiv m-1 \pmod{m},
i.e., the remainder is m1 m-1 when Nk N_k is divided by m m .

10. Assume a a and b b are integers such that 1a<bp 1 \leq a < b \leq p and NaNb(modp) N_a \equiv N_b \pmod{p} . This implies:
1000!pa11000!pb1(modp)    1000!p(ba)0(modp). \frac{1000!}{p}a - 1 \equiv \frac{1000!}{p}b - 1 \pmod{p} \implies \frac{1000!}{p}(b-a) \equiv 0 \pmod{p}.
Since 1a<bp 1 \leq a < b \leq p , 0<ba<p 0 < b - a < p , hence pba p \nmid b-a . Also, p1000!p p \nmid \frac{1000!}{p} because p>500 p > 500 implies 1000<2p<p2 1000 < 2p < p^2 , meaning p21000! p^2 \nmid 1000! . Consequently, Na≢Nb(modp) N_a \not\equiv N_b \pmod{p} .

11. Therefore, N1,N2,,Np N_1, N_2, \ldots, N_p are congruent to 0,1,2,,p1 0, 1, 2, \ldots, p-1 modulo p p in some order. Thus, there is one unique integer c{1,2,,p} c \in \{1, 2, \ldots, p\} such that Nc0(modp) N_c \equiv 0 \pmod{p} .

12. Summarizing, Nc N_c has 999 distinct remainders when divided by 2,3,,1000 2, 3, \ldots, 1000 and one of them is zero. Hence, NcS N_c \in \mathbf{S} , leading to the conclusion:

The values of k k for which it is possible that rk=0 r_k = 0 are all primes between 500 and 1000.

The final answer is all primes between 500 and 1000 \boxed{\text{all primes between 500 and 1000}} .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.