Maxim and Progressions

general
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Sry, I didn’t understand this: Sequence s[1],  s[2],  …,  s[k] is called a subsequence of sequence a[1],  a[2],  …,  a[n], if there will be such increasing sequence of indices i[1], i[2], …, i[k] (1  ≤  i[1]  <  i[2]  < …   <  i[k]  ≤  n), that a[i[j]]  =  s[j]. In other words, sequence s can be obtained from the a by deleting some elements.

that’s mean that if we have the sequence like 1 2 3 4 5 6, so sequence like 1 2 4 6 is subsequence or not? Pls tell me cauze this is the main thing in that problem.

Sry, for my english.

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Yes 1 2 4 6 is a sub-sequence. It is obtained by deleting the elements 3,5. To explain what a sub-sequence is in simple terms, choose any numbers from the sequence and then arrange them in the order they appear. e.g 2 4 3 6 1 10 . Say you choose 1 2 4 10. Then the subsequence will be 2 4 1 10.

3

heard of Google ?

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one more doubt:
if 2 4 6 is one sub-sequence of 1 2 3 4 5 6.
is 2 4 and 4 6 should be consider as new sub-sequence’s or not.?