Low Points of a Sequence

Problem

For a finite sequence E ⁣:a1,a2,…,aME \colon a_1, a_2, \dots, a_M (with M⩾2M \geqslant 2), an index n⩾2n \geqslant 2 is called a low point of EE if ai>ana_i > a_n for every i=1,2,…,n−1i = 1, 2, \dots, n-1. If EE has exactly pp low points, we say EE is a sequence of order pp.

1. Write down all low points of the sequence E ⁣:−1,2,0,−2,4,−5E \colon -1, 2, 0, -2, 4, -5. 2. Prove that if EE has order 00, then a1⩽ana_1 \leqslant a_n for n=1,2,…,Mn = 1, 2, \dots, M. 3. Suppose EE has order kk and satisfies an+1⩾an−2a_{n+1} \geqslant a_n - 2 for all applicable nn. Prove that

a1−aM⩽2k.a_1 - a_M \leqslant 2k.

Answer

Solution

Difficulty6/10
Topicscombinatorics, Logic, algebra, sequences, Extremal Principle, Telescoping

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