Maths Olympiad Prep

Library / /16 of 21

Algebra Difficulty 8.3 Shortlist Find the answer

A natural number nn is given. Determine all (n1)(n - 1)-tuples of nonnegative integers a1,a2,...,an1a_1, a_2, ..., a_{n - 1} such that
m2n1+2m+a12n1+22m+a22n1+23m+a32n1+...+2n1m+an12n1=m\lfloor \frac{m}{2^n - 1}\rfloor + \lfloor \frac{2m + a_1}{2^n - 1}\rfloor + \lfloor \frac{2^2m + a_2}{2^n - 1}\rfloor + \lfloor \frac{2^3m + a_3}{2^n - 1}\rfloor + ... + \lfloor \frac{2^{n - 1}m + a_{n - 1}}{2^n - 1}\rfloor = m
holds for all mZm \in \mathbb{Z}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To determine (n1) (n-1) -tuples of nonnegative integers a1,a2,,an1 a_1, a_2, \ldots, a_{n-1} such that

m2n1+2m+a12n1+22m+a22n1++2n1m+an12n1=m \left\lfloor \frac{m}{2^n - 1} \right\rfloor + \left\lfloor \frac{2m + a_1}{2^n - 1} \right\rfloor + \left\lfloor \frac{2^2m + a_2}{2^n - 1} \right\rfloor + \ldots + \left\lfloor \frac{2^{n-1}m + a_{n-1}}{2^n - 1} \right\rfloor = m

holds for all mZ m \in \mathbb{Z} , we follow the below steps:

1. Rewriting the Floor Function Terms:
Each term in the sum involves a floor function 2km+ak2n1\left\lfloor \frac{2^k m + a_k}{2^n - 1} \right\rfloor. For this entire sum to simplify to exactly m m for any integer m m , the fractional parts must somehow balance out such that overall, we can reconstruct a precise integer result, i.e., bias the floors where needed.

2. Equate Sums and Analyze:
Let us start from the algebraic manipulation:

m=m2n1+k=1n12km+ak2n1 m = \left\lfloor \frac{m}{2^n - 1} \right\rfloor + \sum_{k=1}^{n-1} \left\lfloor \frac{2^k m + a_k}{2^n - 1} \right\rfloor

when rewritten implies:

k=0n12km+ak2n1m2n12n1 \sum_{k=0}^{n-1} \left\lfloor \frac{2^k m + a_k}{2^n - 1} \right\rfloor \approx m \frac{2^n - 1}{2^n - 1}

3. **Determine Specific Values for ak a_k 's**:

As analyzing and checking multiple m m is not trivial without testing boundaries:

- Consider explicitly ak=k(2n1)(2k1)m a_k = k(2^n - 1) - (2^k - 1)m .

Given this choice, compute each step:

ak=(0)(2n1)(201)m=0 a_k = (0)(2^n - 1) - (2^0 - 1)m = 0

ak=(1)(2n1)(211)m=2n1m a_k = (1)(2^n - 1) - (2^1 - 1)m = 2^n - 1 - m

This pattern as it holds till n1 n-1 , confirms that:

ak=k(2n1)(2k1)m a_k = k(2^n - 1) - (2^k - 1)m

Suitably provides non-negative ak a_k satisfying the equation as built when tested via any:

(a1,a2,,an1)=(1(2n1)(211)m,2(2n1)(221)m,,(n1)(2n1)(2n11)m) (a_1, a_2, \ldots, a_{n-1}) = \left(1(2^n - 1) - (2^1 - 1)m, 2(2^n - 1) - (2^2 - 1)m, \ldots, (n-1)(2^n - 1) - (2^{n-1} - 1)m \right)

Thus, the solution to the given problem is:

(a1,a2,,an1)=(1(2n1)(211)m,2(2n1)(221)m,,(n1)(2n1)(2n11)m) \boxed{\left(a_1, a_2, \ldots, a_{n-1}\right) = \left(1(2^n - 1) - (2^1 - 1)m, 2(2^n - 1) - (2^2 - 1)m, \ldots, (n-1)(2^n - 1) - (2^{n-1} - 1)m \right)}

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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