# Can a sequence be arithmatic and geometric?

1. Oct 25, 2006

### tcesni

can a sequence be arithmatic and geometric??

can a sequence be arithmatic an geometric??

2. Oct 25, 2006

### Andrew Mason

Write out the formula for the nth term of each and see if they can be equal for all n for some values for d and r:

$$a_n = a_1 + d(n-1)$$

$$a_n = a_1 r^{n-1}$$

AM

Last edited: Oct 25, 2006
3. Oct 25, 2006

### JasonRox

Yes, they definitely can be equal.

4. Oct 26, 2006

### JasonRox

Did you find one yet?

5. Oct 26, 2006

### GoldPheonix

Well, two sequences can equal eachother at certain points, but a sequence cannot be both geometric and arithmetic. It be a combination of both of them, no doubt, but then it is not arithemetic nor geometric from my understanding of sequences.

6. Oct 28, 2006

### arildno

Sure they can.
The general n'th term in an aritmetic sequence is
$$a_{n}=a_{0}r^{n}$$
whereas the general term in a geometric sequence is:
$$g_{n}=g_{0}+kn$$
where $a_{0},g_{0},r,k$ are constants independent of n.
A sequence that is both arithmetic and geometric fulfills BOTH conditions for all choices of n, which means that we must have:
$$g_{0}+kn=a_{0}r^{n} (*)$$
That is, we must have $g_{n}=a_{n}$ for all n.
Now, you might fiddle about to find constants $a_{0}, g_{0},k,r$
so that (*) holds for all n. It can be done.

Last edited: Oct 28, 2006
7. Oct 28, 2006

### HallsofIvy

Staff Emeritus
Uh, Arildno- you have the definitions of "geometric" and "arithmetic" sequences reversed.

8. Oct 28, 2006

### Office_Shredder

Staff Emeritus
The trivial example is 1, 1, 1, 1, which is both arithmetic with respect to 0 (I don't think that's proper terminology, but I'll be damned if it doesn't sound good) and geometric with respect to 1

9. Oct 29, 2006

### arildno

Uhmm, blarrg, huge embarassment.
Please tell me when I can take my head out of the bucket.

10. Oct 29, 2006

### HallsofIvy

Staff Emeritus
Actually, the sequence a, a, a, a, ..., for any a, is both arithmetic (a+ 0n) and geometric (a(1n)). It's easy to show that any sequence that is both arithmetic and geometric must be of the form a, a, a, a, ... for some a.

(You may now remove head from bucket, arildno.)

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