Maths Olympiad Prep

Library / /70 of 84

, 2014

Algebra Difficulty 5.8 AIME, harder Prove it United States

Problem:

Let f:NNf: \mathbb{N} \rightarrow \mathbb{N} be a function satisfying the following conditions:
(a) f(1)=1f(1)=1.
(b) f(a)f(b)f(a) \leq f(b) whenever aa and bb are positive integers with aba \leq b.
(c) f(2a)=f(a)+1f(2 a)=f(a)+1 for all positive integers aa.

How many possible values can the 2014-tuple (f(1),f(2),,f(2014))(f(1), f(2), \ldots, f(2014)) take?

Solution

Solution:

Answer: 10071007

Note that f(2014)=f(1007)+1f(2014)=f(1007)+1, so there must be exactly one index 1008i20141008 \leq i \leq 2014 such that f(i)=f(i1)+1f(i)=f(i-1)+1, and for all 1008j20141008 \leq j \leq 2014, jij \neq i we must have f(j)=f(j1)f(j)=f(j-1). We first claim that each value of ii corresponds to exactly one 2014-tuple (f(1),,f(2014))(f(1), \ldots, f(2014)). To prove this, note that f(1024)=11f(1024)=11, so each ii uniquely determines the values of f(1007),,f(2014)f(1007), \ldots, f(2014). Then all of f(1),,f(1006)f(1), \ldots, f(1006) can be uniquely determined from these values because for any 1k10061 \leq k \leq 1006, there exists a unique nn such that 1007k2n20141007 \leq k \cdot 2^{n} \leq 2014. It's also clear that these values satisfy the condition that ff is nondecreasing, so we have a correspondence from each 1008i20141008 \leq i \leq 2014 to a unique 2014-tuple.

Also, given any valid 2014-tuple (f(1),,f(2014))(f(1), \ldots, f(2014)), we know that f(1),,f(1006)f(1), \ldots, f(1006) can be uniquely determined by f(1007),,f(2014)f(1007), \ldots, f(2014), which yields some 1008i20141008 \leq i \leq 2014 where f(i)=f(i1)+1f(i)=f(i-1)+1, so we actually have a bijection between possible values of ii and 2014-tuples. Therefore, the total number of possible 2014-tuples is 10071007.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.