MHB Is a number member of sequence?

  • Thread starter Thread starter cfg
  • Start date Start date
  • Tags Tags
    Member Sequence
AI Thread Summary
The discussion revolves around determining if the numbers 10 and 6 are members of the sequence defined by the equation an(n in subindex)=(1/2)*n^2-3n+5/2 for n ≥ 1. To find this, participants suggest solving the equations derived from setting the sequence equal to these numbers. The quadratic nature of the sequence implies that solutions can be found by analyzing the discriminant for integral roots. The discussion also touches on the recursive definition of the sequence, which adds depth to the analysis. Ultimately, the focus is on solving the equations to confirm membership in the sequence.
cfg
Messages
1
Reaction score
0
an(n in subindex)=(1/2)*n^2-3n+5/2, when n ≥1

Is number 10 member of that sequence? what about number 6?Create equation to solve it.

If someone can help with this problem please, it will be much appreciated!
 
Mathematics news on Phys.org
cfg said:
an(n in subindex)=(1/2)*n^2-3n+5/2, when n ≥1

Is number 10 member of that sequence? what about number 6?Create equation to solve it.

If someone can help with this problem please, it will be much appreciated!
Is that $\frac{n^2}{2}-3n+\frac{5}{2}$?

If so, then if 10 is a member of the sequence, then there is a positive integer n that satisfies $$\frac{n^2}{2}-3n+\frac{5}{2}=10$$
Either find such a solution to the equation (solve for n) or prove that there isn't one. Do the same for 6.
 
While this may be beyond the scope of what is expected or even needed here, we could observe that since the closed form of the sequence is a quadratic with real coefficients, then the recursive form will come from the characteristic equation:

$$(r-1)^3=r^3-3r^2+3r-1$$

and so the sequence may be defined recursively as:

$$a_{n+3}=3a_{n+2}-3a_{n+1}+a_{n}$$

where:

$$a_1=0,\,a_2=-\frac{3}{2},\,a_3=-2$$

Another thing we might look at is the equation:

$$\frac{n^2-6n+5}{2}=a_n$$

$$n^2-6n+5-2a_n=0$$

If there is going to be an integral root, then the discriminant will be a perfect square, the square of an even number:

$$(-6)^2-4(1)\left(5-2a_n \right)=(2m)^2$$ where $$0\le m\in\mathbb{Z}$$

$$9+2a_n-5=m^2$$

$$4+2a_n=m^2$$

$$2\left(2+a_n \right)=m^2$$

So, we can easily see that when $$a_n=6\implies m=4$$. What about when $a_n=10$?
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...

Similar threads

Replies
8
Views
3K
Replies
1
Views
2K
Replies
11
Views
2K
Replies
1
Views
2K
Replies
7
Views
2K
Replies
55
Views
5K
Back
Top