What is the formula for the $n$th term of the interesting sequence?

  • Context: MHB 
  • Thread starter Thread starter alexmahone
  • Start date Start date
  • Tags Tags
    Interesting Sequence
Click For Summary

Discussion Overview

The discussion revolves around finding the formula for the $n$th term of a specific sequence defined by the integers being included a number of times equal to their value. The focus is on deriving a mathematical expression for the term based on its position in the sequence, with hints provided for an assignment problem.

Discussion Character

  • Exploratory, Mathematical reasoning, Homework-related

Main Points Raised

  • One participant proposes that the $n$th term of the sequence can be expressed as $a_n=\lfloor\sqrt{2n}+\frac{1}{2}\rfloor$.
  • Another participant suggests considering when the $k$th integer first and last appears in the sequence, indicating a relationship with triangular numbers.
  • A subsequent post clarifies that the integer $k$ first appears at the $\left[\dfrac{(k-1)k}{2}+1\right]$th position and last appears at the $\left[\dfrac{(k-1)k}{2}+k\right]$th position.
  • Another participant notes that the sequence increments at positions of the form $n = (k - 1)k/2 + k = k(k + 1)/2$, suggesting a connection between this pattern and the proposed formula.

Areas of Agreement / Disagreement

Participants are exploring the relationship between the sequence and triangular numbers, but there is no consensus on the correctness of the proposed formula or the next steps to derive it.

Contextual Notes

The discussion includes hints and partial reasoning but lacks complete resolutions or definitive conclusions regarding the formula's validity.

Who May Find This Useful

Readers interested in mathematical sequences, number theory, or those working on related homework problems may find this discussion relevant.

alexmahone
Messages
303
Reaction score
0
Let $a_n$ be the $n$th term of the sequence $1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, ...,$ constructed by including the integer $k$ exactly $k$ times. Show that $a_n=\lfloor\sqrt{2n}+\frac{1}{2}\rfloor$.

(Hints only as this is an assignment problem.)
 
Physics news on Phys.org
Hint: when does the $k$th digit first and last appear in the sequence? Use the triangular numbers.
 
The integer $k$ first appears at the $\left[\dfrac{(k-1)k}{2}+1\right]$th position and last appears at the $\left[\dfrac{(k-1)k}{2}+k\right]$th position.

Not sure where to go from here.
 
So that tells you that the sequence increments immediately after every index of the form $n = (k - 1)k/2 + k = k(k + 1)/2$, and nowhere else, right? So if you can show that the expression you are given $\sqrt{2n} + 1/2$ increments for exactly the same $n$'s, then the floor of that must be equivalent to your sequence.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 6 ·
Replies
6
Views
4K